She was a Spencer, adored by all but her husband. She was beautiful, but plagued by bulimia. She lived 200 years ago, yet Georgiana`s life uncannily paralleled that of her famous Twentieth-Century descendant, Diana.

Standing under the gilded octagonal dome of Chiswick House with Amanda Foreman, a 29-year-old blonde in a biker jacket and jeans, I watch her trying, for my sake, to summon the ghost of someone who once lived here. She is a historian with a sense of theatre. Sticking her hands into her pockets, she paces the ground slowly, and talks me in. "Chiswick was just a country cottage to her", she says. "Her husband owned seven estates, including Chatsworth, Burlington House and Devonshire House. But Chiswick was her favourite. She called it her "earthly paradise".

"Even as I talk to you, I can feel her so near to us", she says. "Each time we enter a room, she has just left it ...

"Now you can hear children's voices, laughter and running feet. Here she comes, through this door! I can see part of her face. It is a lovely face, with those slightly bulging Spencer eyes, and marmalade-gold hair loosely piled up on her head. She is wearing a white muslin dress with a loose drawstring ruff, and a pale blue satin sash, and diamonds and pearls. She has children running round her skirts, and she is talking to them. She's like the Pied Piper; she's being followed by a group of people all trying to talk to her and hear what she's saying. She turns back and says something, and they all laugh. Now she's going through that door, with everyone hurrying after her ... and now the footsteps are dying away".

"Where is she going?"

"To 'nuncheon' in the garden. In a minute, when she arrives, you will hear a tremendous buzz of excitement, and then the conversation with begin".

So vividly has Amanda Foreman brought her to life that it is the fate of the young biographer to be hopelessly upstaged by the subject of her marvellous first book, Georgian, Duchess of Devonshire, who in 1774 married one of the richest men in England, and became, as Foreman argues, an eminence grass in the Whig party. She had some extraordinary similarities to Diana, Princess of Wales, but there was more to her than popular icon or fashionable beauty.

The 405 pages leave the reader feeling that ours is a pretty dull century. In terms of our anaemic modern society, the 18th-century duchess was Princess Diana and Mick Jagger and Tony Benn all in one. To have been part of Georgiana's circle would have provided all the thrills, gossip and rock'n'roll that you could want to experience at first hand. Georgiana lived in a menage-a-trois, was the lover of a prime minister, had an illegitimate child, accrued gambling debts to the equivalent of £3,720,000 today, and originated the role of political press agent, and lived during the madness of George III, the French revolution, the execution of her close friend Marie Antoinette, and the Irish rebellion.

Foreman has written a definitive book - entertaining, academically precise and, in the words of Dr. Leslie Mitchell of University College, Oxford, the premier Whig historian and biographer of Charles James Fox, "the first book on the subject which has called on every surviving piece of material and presents the total picture".

With "George-ayna", as she was called, we enter into every eccentric corner of Devonshire House society, satirised by Richard Brinsley Sheridan in The School for Scandal, and where the newly married Georgiana is depicted as the spendthrift leader of fashion, Lady Teazle. The circle even had its own dialect, called the Devonshire House drawl. Hope was "whop", yellow was "yaller", and spoiled rhymed with mile. Their affectations were widely parodied, and to some extent infiltrated the speech of the Whig party. "Dearest one, how do oo do?" the ladies might whisper to each other, when they meet in the corridors of Chatsworth and answer, "As oo do, so does poor little I".

During her five years of painstaking research, the young Anglo-American academic found herself at times overwhelmed by the strength of a character who has been dead for nearly 200 years. In the last six months of writing, Foreman had no other life, working six days a week from 6 am to midnight, and when the book was finished, she suffered an extreme form of psychological deprivation. Today she still looks wan and fragile, and her large brown eyes have an unusual intensity.

"Georgiana's presence and her voice took me over", she says, as we walk towards the portico through the red and blue velvet rooms. "My real life and relationships paled in comparison. I had fierce, troubling dreams about her at night. I would be chasing her through a series of gilded rooms, like this one, and every time I caught up with her, she would have a different face".

In the car park, we climb into her Peugeot 205, clearing a space between the broken sunglasses, crumpled papers, sweet and cigarette packets, Biros and parking tickets. We pull out smartly across the Great West Road, and twice escape death by lorry as she gives me a recondite introduction to the Namierite interpretation of history. Turning into Fulham, she gives me a short tutorial on why her biography is a blast against the "separate spheres" ideology of 1970s feminism, and misses the back bumper of a taxi by a squeak.

"My sister drives like a character in an Evelyn Waugh novel", says her 32-year-old brother, Jonathan, leader writer for the New York Post. "She took her test seven times. I don't know how she survived those 18 months on the road when she was researching the book".

The day she finished the book, a terrible loneliness came over her; and after the depression came anger. She felt that something had been taken from her, and while her attention had been fixed on another woman, her boyfriends had drifted away.

"I fell in love with Georgiana", she admits. "And it has been the most important relationship of my life".

Her brother watched her obsession with some bewilderment. When she discovered a lost letter exposing for the first time the betrayal of Georgiana by her lover Charles Grey - later Earl Grey, the prime minister responsible for the Reform Bill, which opened up voting to the middle classes - and found that in fact another woman had been the love of his life, foreman wept all the way home on the Underground from the Public Record Office in Kew. "She was devastated for a whole week," Jonathan says. "It was alarming. I would tell her, "Steady on, aren't you getting a bit too involved in this?"

Back at her decorative house in Fulham, she throws her leather jacket across a chair, pushes her spectacles up into her hair, and shows me the rickety card table where she worked on her laptop, surrounded by needlepoint rugs and cloth-covered tables, Madonna lilies and jardinieres filled with plants. "Would you like to see the garden?" I am surprised by the question which is so normal and middle-class. "Oh yeah, I nearly did domestic science and studied flower arrangement", she says. "My mother did a wonderful thing. She said, 'No way. You will go to university if it kills me.'

"I was hopeless at school, you see. My brother got perfect A-levels and a scholarship to Cambridge, and I was a terrible disappointment. It was mortifying. My father found it puzzling that I couldn't pass an exam. He would ask me, 'Why are you failing maths?'

"My mother is stylish and charming and my father was charismatic. Whenever they visited me at school, there was electric interest in them. There was this troubled, self-conscious, underperforming, screwed-up and unhappy kid. Apart from anything else, I was bored out of my mind".

A spark was lit for the first time when she read Plato's Republic, aged 15. Desperate to find someone she could talk to about it, she timidly approached a teacher. "He said I should write an essay, then he would discuss it. When I handed it in, he just ringed the mistakes and told me to come back when I had learnt to spell." The flame was extinguished for another three years.

She was left-handed and her writing was illegible. She failed her A-levels and the retakes. She got an E for English. She was unpopular and victimised at school. "When I went to Sherborne, I had an American accent and a sign over my head saying, 'Bully me'! She laughs, but there is pain in the laughter. "I was the school freak, and I discovered that when you're bullied, the teachers gang up on you too. It was deeply humiliating. My father was not a man you would wish to disappoint".

There is a framed photograph on her drawing-room wall of her father, the Hollywood screenwriter Carl Foreman, with Sir Winston Churchill. Foreman leans forward, talking intensely, unaware of the camera - an attractive, grey-haired man with spectacles, full of energy. He died of cancer when she was 16.

She tried to get into university in England for two years running, and was turned down 12 times. Her English mother, now Eve Williams-Jones, then took her on a 10-day whistle-stop tour of American colleges, with no success. She had been turned down by no fewer than 26 establishments by the time her mother took her to Sarah Lawrence, an alternative college in New York.

"Finally, no exams, no grades. You have a contract with your professor and design your own coursework, and at the end of the day it's all about whether you have been fulfilled or not.

"I wasted half a term being furious. I thought, if they will take me, this must be the world's worst college! Then I decided that whatever else happened, I wouldn't add to my chapter of failures."

Quite suddenly, she shone. Before the end of the first term she had been elected head of the student union, and wet on to become a brilliant student of philosophy, psychology and history. She became an expert on Wittgenstein and the German idealists. She took a guest studentship at Columbia for a year, "because I had to know - could I do as well in a normal university?" and got straight As al through. Back at Sarah Lawrence, at weekends she compiled, printed and distributed the first directory for volunteers of charity organisations within a radius of 300 miles.

"I think she was bound to do better over her," says her main teacher at Sarah Lawrence, Professor Frederic Smoler. "In England you think a few people are clever and all the rest are stupid. In America, we think every fool has a right to his opinion".

She then applied to Oxford to an undergraduate degree. Accepted by Lady Margaret Hall in 1991, she worked for six weeks before it became clear that she was going to get a first, when she was upgraded to an Mphil postgraduate degree. Her thesis was The Politics Behind the Abolition of the Slave Trade in 1806-07. It won the Henrietta Jex-Blake Graduate Scholarship twice over.

Once she got to grips with the 18th century, she had the Furies behind her. "While other people were at parties, Amanda would be working in the library", says Dr. Mitchell. "Later, when her friends were getting married, she would be digging away in libraries like a terrier. It's a strange way to spend your 20s, but history is important to her".

Why? "Because history cannot but have an influence on your world view and your sense of future," Foreman says emphatically. It is an answer she tried to din into every undergraduate she taught when she was working for her postgraduate degree. "History always plays a role in politics, and politics is always about righting historical wrongs".

Her brother, who calls her Bill - "Amanda ... Manda .... Mandible .... Bill" - says that, at 17, at the flower-arranging stage, she had given up. "She went all Sloaney and said she wasn't clever, she was going to get married. When she had academic success at last, it was all the more important to her. By the end of Oxford, she had become fiercely intellectual and terribly serious. The book has relaxed all that, softened and broadened her. From being interested in Georgiana as a leader of the Whigs, she became equally interested in her as a person."

So what, I asked her, had gone wrong with her early life? Was it connected with a strange sentence at the end of the acknowledgements that says, "I dedicate this book to my father, Carl Foreman, in the certain knowledge that if he had lived, by now we too would be friends"?

"I just wish I had been born 10 years old. I would have had him up to the age of 25, and I would have got to know him better, and we could have had real conversations. As it was, I didn't understand or know him". She speaks sadly. "I'm sure, in the end, I would have made him proud of me".

Carl Foreman was a great screenwriter who wrote the classic movies High Noon, Home of the Brave, The Guns of Navarone, The Bridge of the River Kwai, Born Free and Young Winston - which led to the photographed meeting with Churchill. In the 1950s he was caught in the rise of McCarthyism. A paranoid Congress obsessed with communism blacklisted him for refusing to co-operate. Subpoenaed to name names before the House Committee on Un-American Activities, he abruptly left America with his family, arriving in Britain without money or possessions. He did not go back until a crisis in the British film industry coincided with an American invitation to return, in 1973. The upheavals left the family with American passports and a nagging sense of insecurity. "After he died, I discovered he had bank accounts in every country. If it had ever come again, and we'd had to run, there would have been an open bank account to go to, with a little money in it," she says.

Foreman was nominated for an Oscar eight times. "When I was 10 I asked him if he had ever won it. He said that he had, theoretically, for The Bridge on the River Kwai, but that, as he had written the screenplay anonymously, they had given the Oscar to the writer of the original book, Pierre Boulle. I said to him, 'Why don't you ask them for it?' He said, 'No, I'm waiting for them to give it to me'. It wasn't until after his death that his name replaced that of the author on the credits.

"He came from a long line of rabbis. When he was 14 the family went bankrupt. It was in Chicago, during the Depression. they were getting poorer and poorer, and his mother was making hats to keep food on the table. But his father would leave the house as usual, at 8 in the morning, and come back at 6 pm, and my father assumed he was in work. Then, one day, he was walking back from school and he saw a man selling matches in the street. It was his father."

The scenes roll back, as she speaks, on generation after generation of her family in Russia and America - each subjected to the worst that could happen.

When Carl Foreman's mother, Fanny, was a child, her family walked out of their front door in Moscow and kept on walking until they reached Odessa, where they escaped by boat to the United States.

Amanda Foreman's personal history explains her political passion and intensity. Her friends call her Phoebe, after the character from Friends, probably on account of her wide Russian cheekbones. "Is it?" she asks, frowning with worry. "We're not at all alike. I mean, she's very strange. What does that mean?"

When her father first reached Hollywood, he lived in the open up in the Hills until he got a job storing movie reels. He was such a promising writer that when he dropped out of screenwriting school for lack of money, the principal paid the fees out of his own pocket, pretending to Carl that he had won a scholarship. The screenwriter believed him, probably because he didn't want to know about failure. "There was a competition. You submitted your screenplay, and the winning script would be made into a film. My father submitted two scripts anonymously, and won first and second prizes.

"He was 70 when he died, and there is so much he said to me that I have only understood later. For instance, I remember him saying, 'Don't confuse ambition with snobbery' and 'Don't confuse success with social success.' Recently I have incorporated that into my world view. In my 20s I would join any club that would have me. The premises about clubs is that there is no point in being included unless others are excluded. I don't join them any more".

"My daughter has always been a freethinker", says Eve Williams-Jones, who is now married to the recently retired president and chief executive officer of the film distribution company UIP. "She became president of the student union just as the campus revolutions began". A caucus of righteous students took over the campus, claiming the right to censor the college newspaper for reasons of political correctness. They determined to take over the hiring and firing of professors on the basis of racial and ideological attitudes. "It was a frightening time, but she refused to be intimidated. If I had to describe her in a word I would say she had tenacity".

Professor Smoler remembers Foreman's "moral vigour and huge courage". She stood firm against an idiotic Left and was fearless when some of the students occupied the college for six weeks. Not that she was particularly right. If you are the daughter of a man who has paid a real price for his freedom, you don't fall for the posturing of people who are pretending to pay for imaginary freedoms. Argument is usually the first casualty of excitable times, but she continued to engage in energetic and frank debate." Today she is a fundraiser for Index on Censorship, the magazine for free speech started 25 years ago by Stephen Spender and Harold Pinter to provide a forum for censored voices. She is also foreign editor of The Week. Does she do anything other than work? "I can't get enough of science fiction; there's not an instalment of ER or Frasier I haven't seen, and I have total recall of The X-Files, Babylon 5 and Star Trek". She also goes to all the movies.

"What did you think of Titanic?"

"It was posited in such a lazy way. I mean, it was turned into a massive Irish jig. The rich all behaved badly and the poor were all Irish and locked down below. The really interesting think about the Titanic is that there were 300 married couples on board, and 297 women elected to leave their husbands behind."

Last night, she reread half of Pride and Prejudice - "Mind candy!" I say that it is hard to believe that Jane Austen was a contemporary of Georgiana's, even if she was 20 years younger.

"Remember Play School? If you look through Jane Austen's round window you see a completely different 18th century from what you see if you look through Georgiana's square window. The women don't do anything. The men don't do anything. I mean - there was a war going on!" She is so fired up with the arguments that she sometimes gives the impression of a one-woman debating society.

"In the 18th century, everyone was acting out their emotions. Now, Georgiana would just say, 'I have an obsessive-compulsive disorder and an addictive personality.' And we would say, 'Why?' And she would say, 'Because I had an emotionally depressed and scary father, and an overprotective and manipulative mother.' And if she wondered why she found herself loving her husband's mistress with a strange intensity, she would ask herself, in the words of a Cosmo-cover, 'Why am I other-directed?'

Was she a lesbian?

"Not per se. She enjoyed sex with men. So the question is, was she bisexual? Does it matter?" It's like being in the company of a charismatic teacher.

"I wasn't much good as a teacher. It broke my heart. My students didn't know why they were leaning all this stuff, they didn't care and they couldn't be bothered to put in the hours. I mean, one of the great questions in 18th-century history is whether there was a Tory party after 1714. It matters because these questions are perennial Will we have a Tory party in 10 years' time?"

But I have a much more interesting subject to ask her about. Has she noticed the extraordinary number of parallels between Georgiana and Diana, Princess of Wales? They were both Spencers who married much older men and ended by eclipsing their husbands in the public eye. It was said of both of them that their spouses were the only men in England who were not in love with them. Like Diana, Georgiana had an eating disorder, was devoted to her charities, went through an early stage of hoydenish behaviour, and led fashion. they were both targeted by the press for their style and glamour. Georgiana carried fashion to extremes that would have made Galliano gasp, sometimes decorating her 3 ft tower of hair with a ship in full sail, or a farmyard of wooden animals. At the opera, when the singer Lavatini came on stage wearing a parody of her headdress, the audience roared with laughter as she stood up in her box and swept him a deep bow.

Foreman's blank expression betrays a considerable lack of interest in the subject. For the first time, she hasn't much to say.

"But they were both the most famous women of their day ... They returned after huge setbacks .... They were adored by the press, and they loved the attention!" I say. "I haven't really thought much about it. Don't you think we're all original and different? I don't want to reduce the uniqueness of Georgiana's story .... or Diana's".

I have a feeling that I am looking through the round window, while Amanda Foreman is looking through the square window.

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