Nevertheless, within a week of being chucked, Mitford, about to turn the frightful age of 30, was engaged to another man - the Hon. Peter Rodd, aka "the Prod", handsome, clever, lazy, cruel - a person Mitford would later describe as "the most boring man in the world". "He had proposed to numerous women - on one occasion, two in a single night - before Nancy accepted him at a party, and though he wrote to her suggesting squirmingly that the proposal had merely been a joke, Nancy for once refused to see it," writes Hilton. "Peter represented pretty much her last chance, and she knew it."
It was not a happy marriage, blighted by the Prod's drinking and financial recklessness (he took to thieving from his wife, who had started to make money as a writer) and Mitford's miscarriages and subsequent hysterectomy.
He took a mistress, a married woman who was a "friend" of the Rodds, and the couples played bridge together. "Peter flirted, Nancy sniped and when the atmosphere became too gruesome [she] would pretend to faint," says Hilton in the book. "Peter would dump her on the drawing room sofa and go back to the game and his lover."
No wonder Mitford was ripe for romance in the unlikely form of General de Gaulle's chief of staff, Palewski. He had set up base in London during World War II as part of the Free French movement, the opposition to the pro-Nazi Petain Government. Mitford first met him in London in 1942, and was immediately swept away by feelings she later described in her novel The Pursuit Of Love: "Linda [her heroine] was feeling ... an overwhelming physical attraction. It made her quite giddy, it terrified her."
Apparently Palewski oozed charisma, humour and intelligence. Physically, he sounds vile. "Gaston had acne-pitted skin, 'a face like an unpeeled King Edward', receding hair and ... halitosis that could stop traffic," writes Hilton, who went to Paris in 2009 to meet a group of ladies "of considerably more than a certain age" who had known the colonel.
"Ooh, they were fabulous," she says, laughing. "They were such fun. They said he was 'built for love' and that Englishmen were 'hopeless'. What was so interesting about him and which comes across so sparklingly well was that he was a man who liked women. He was a seducer of women" - Hilton writes that Palewski "operated on a principle of maximum returns, making passes at practically every woman he met".
"Gaston liked women - the Mitfords' favourite occupation was chatter and Gaston loved to chatter. He liked jokes, he loved poetry, he loved women's company. I think what was so unusual about that was, again, the worlds of the sexes, particularly in that class. In England the sexes were very much segregated. Men went to university, they went into their professions and spent time at their clubs.
"The French have always considered that segregation as a barbaric custom. They have always valued the intellectualism of women and, for someone like Nancy, this was a breath of fresh air. It's very notable that all the men she was drawn to as friends and even potential lovers in her 20s were gay men. So to find a man who was fantastically macho, a decorated colonel, a fighter pilot, the politician who also had a very feminine side, was really intoxicating."
Mitford moved to Paris after the war ended, living in the same city as her lover but pursuing an independent life as a writer. They never lived together and he never even stayed the night. The Prod refused to give her a divorce for many years, and "as long as she was married she didn't have to face up to the fact Gaston would not marry her if she was free", writes Hilton.
Palewski was a roue through and through. He continued to pursue his principle of maximum returns, with vulgar behaviour, which has seen Mitford cast by previous biographers as the pathetic little woman who waited at home for his calls. One episode in particular casts a cold light on their relationship, when Nancy, Peter (they had mellowed into a congenial friendship) and his nephews were wandering around Paris one evening and kept banging into Gaston, hand-in-hand with one of his young girlfriends.
"Later, she explained to Diana [her sister] that what she couldn't bear was that he had looked happy, 'so dreadful to prefer the loved one to be unhappy ... Oh the horror of love'," quotes Hilton in her book.
"She was intensely romantic and intensely realistic," Hilton says. "She saw Gaston for what he was and, over a period of time, she negotiated with herself with what she could live with. She decided she could accept the way he was and that he gave her enough happiness to make it worth while even though she did suffer, yes."
Hilton disputes the suggestion, disseminated in previous Mitford biographies, that the relationship was all on Palewski's terms. "When I read the letters which Charlotte Mosley [Nancy's niece-in-law] gave me access to, one has a sense of a relationship between equals. She writes to him about her work, getting his advice, she takes time away from him to write a book - you get a sense of a very mutually supportive relationship, two people who admired one another."
However, he didn't admire Mitford enough to marry her, instead telling her - just before the news broke in Le Figaro in 1969 - that he was getting married to a woman he'd been involved with for nearly two decades, a Polish duchess who owned Le Marais, a beautiful chateau near Paris he coveted more than anything. The marriage quickly became notorious for its coldness.
Mitford died of cancer about a year later, not of a broken heart as some have deduced, but because of medical incompetence and misdiagnosis, says Hilton. Mitford and Palewski had continued to write to each other since his marriage and, by pure intuition, he went to see her in hospital on the day she died. She was unconscious but, Hilton writes, "when he spoke to her and held her hand, she smiled".
A late chapter in the book reveals an unpleasant aspect of Palewski's career. In the 1960s, de Gaulle appointed him Minister for Scientific Research, Atomic Energy and Space, and he initiated the first nuclear tests at Mururoa. He died in 1980, "not comforted by a loving and supportive family as the end drew near".
The Horror Of Love (Phoenix $29.99) is out now.
