Diana Mosley was born a Mitford, married a Guinness and eloped with Sir Oswald Mosley. There can have been few lives in the 20th century quite so controversial or full of contrasts. Her early days typified the existence of the Bright Young Things, and none shone more brightly or mercurially than Diana. But in the 1930s she allied herself to Oswald Mosley and from then on her name was inextricably linked with Fascism.

Mosley created the British Union of Fascists, and Diana supported him wholeheartedly. She followed him into prison at the beginning of the Second World War and into voluntary exile in France after it. She lived long enough to witness a revival of historical interest in the Mosleys by a new generation, and she retained a faithful band of non-political friends, who loved her for her wide-ranging knowledge of literature, literary and artistic characters, and for her good-humoured anecdotes.

The Hon Diana Freeman-Mitford was the fourth child and third daughter of David Mitford and his wife, the former Sydney Bowles. She was born in 1910 and educated at home until she was 16. Her father succeeded as the 2nd Baron Redesdale in 1916. Diana’s early life was spent at Malcolm House, on the Batsford Park estate, the home built by her grandfather, and memorable for its large library and many oriental treasures acquired on visits abroad. Her father sold Batsford after the First World War and moved his family to Asthall Manor in Oxfordshire, which was cheaper to run.

His relative lack of funds caused him to create a carnivalesque lifestyle, and Diana grew up in the world of “child-hunts”, rages, pets and various odd moneymaking schemes initiated by her father. The atmosphere was memorably captured in at least three of Nancy Mitford’s novels, Highland Fling (in which her father was depicted as General Murgatroyd), In Pursuit of Love and Love in a Cold Climate (in which he was Uncle Matthew). Diana was portrayed as Linda in the two later novels.

She was the most beautiful of a good-looking group of sisters. Her elder sisters were Nancy, the novelist, and Pamela Jackson, the least well-known. Younger than her were the controversially pro-Hitler Unity, the Communist Jessica, and Deborah or “Debo”, who became the Duchess of Devonshire. Her elder brother, Tom, was killed in Burma in 1945. The sisters possessed a quality well defined by Logan Pearsall Smith: “Hearts that are delicate and kind and tongues that are neither — these make the finest company in the world.”

Growing up, Diana was much influenced by the intellectual friends of her sister Nancy and her brother Tom. In 1929, when still only 18, she married the Hon Bryan Guinness, later 2nd Baron Moyne, a rich scion of the Guinness family. They settled at Biddesden House, in Hampshire, and she found herself in the midst of many of his Oxford contemporaries, men who contributed much to the literary and artistic life of the era. She became friends, for instance, with Lytton Strachey, Cecil Beaton, John Betjeman and Dora Carrington. Evelyn Waugh dedicated Vile Bodies to her. Her portrait was painted by Augustus John, Pavel Tchelitchew and Henry Lamb.

She burgeoned into a leading London hostess, beautiful, intelligent, well-read and original. With Bryan Guinness she had two sons, Jonathan (the present Lord Moyne) and Desmond, co-founder of the Irish Georgian Society.

All the Mitford sisters suffered from a strong element of self-delusion, moulding a story until it suited them and then adhering to it with fanaticism. Unity became obsessed by Hitler, Jessica by communism, Nancy fell for Gaston Palewski and moved to France to be near him. In February 1932, Diana met Oswald Mosley, then married to Lady Cynthia Curzon. He had lately left the Labour Party to found the British Union of Fascists. He and Diana discussed politics, and she told him that she had felt furiously sympathetic to the miners during the 1926 National Strike.

Mosley pursued her with vigour and won her love by telling her that only she could help him to achieve his objectives. She espoused his cause and began a blatantly public love affair with him, becoming his mistress and leaving her husband. As a recent biographer put it: “Only someone supremely confident in her ability to survive in a social world hostile to divorce, and uncaring of family and public opinion, would have abandoned a man so kind, goodlooking, loving and rich, by whom she had two children.” Mosley’s wife, Cimmie, died not long afterwards, in May 1933.

Diana found Mosley a captivating personality, older, more impressive and compelling than the smart set she entertained. She found him unusual, and though he engaged in other affairs — notably with his sister-in-law, Lady Alexandra Metcalfe — Diana remained steadfast and eventually won him for herself. The British Union of Fascists began to hold rallies, which she attended, sitting quietly in the crowd even when the rallies became dangerous. Through illness, she missed the famously bloody one at Olympia, and later she said she wished she had been there, confident, she always insisted, that the press had exaggerated their accounts of it.

When Hitler came to power, Diana and Mosley attended the famous Nuremberg rally, presently making further trips to Germany and becoming acquainted with the leading Nazis. In 1935 she at last met Hitler. She did not fall for him in the way that Unity did, but met him many times, often in the cause of fundraising for Mosley’s party in Britain. She caused considerable offence in her postwar memoirs by dwelling on Hitler’s more positive points: his exceptionally fine brown hair, his well-shaped hands, his humour and ability to imitate people amusingly, his politeness to women.

In 1936 Diana and Mosley were married secretly at the house of Dr Goebbels, with Hitler in attendance. News of the marriage was kept from family and friends until November 1938. This marriage took her further into the political arena, and eventually away from a position in British society in which she would have dazzled for many decades. Her husband wrote in later life that his one regret was denying her that role.

War broke out in 1939, and Oswald Mosley was soon interned under Defence Regulation 18B. On July 1, 1940, Diana, by then the mother of two Mosley boys, Alexander and Max, was likewise imprisoned as a danger to the State, and taken from 11-week-old Max, whom she was still nursing. She was locked up in F Block of Holloway Prison. Later in the war, Churchill allowed the Mosleys to be interned together.