The English aristocracy has had its fair share of misfits, and one of the most far-fetched was Unity Mitford. No novelist would dare invent the story of a young woman of 19 who settles in Germany in 1933, determines to captivate Hitler, and succeeds. Eva Braun, the long-term mistress whom Hitler married in the last days of his life, gives way in her diary to jealousy and spite. There is evidence provided either by Unity herself or Nazi officials that Hitler held her hand, stroked her hair and called her ‘Kind’ (child). During his preparation for world war in the summer of 1939, he found time to arrange for a Jewish couple to be dispossessed from their apartment in Munich in order for Unity to have it. He also paid various expenses.

My biography of Unity began when Jessica Mitford gave me some letters written by her sister, and described how Unity and Hitler had been in the habit of drawing up lists of English men and women to be summarily shot after the Germans had invaded and conquered the country. Was Unity a genuine Nazi, or merely fantasising a Vichy England in which she would come into her own? Could she have imagined herself as the Hon. Mrs Adolf Hitler? Jessica had also done her best to be a misfit. I went to Paris to interview the more astringent Nancy Mitford, for whom her family was a running comedy of manners. I should write the book, she said, adding that the sisters wouldn’t like it.

First came Lady Mosley (née Diana Mitford). She told me that I was too young to understand Unity or Nazism, but would live long enough to see statues of Hitler and Goebbels in the capital cities of Europe. Demonstrators today holding placards with the slogans ‘Hitler was right’ and ‘Jews to the gas’ may be proving her prophetic. My Oxford tutor, the historian A.J.P. Taylor, admired Sir Oswald Mosley, often dined with him and one day had me invited too. Mosley had married in Berlin in the presence of Hitler and Goebbels and of course his sister-in-law Unity; he had accepted subsidies from Goebbels’s ministry and been well placed for the Pétain role in a Vichy England. Eyes bulging, he lied effortlessly about his past. The Duchess of Devonshire (née Deborah Mitford) refused to meet me, but wrote to my father with a command adapted from training dogs, ‘Call your boy off.’ He answered that I was nearly 40. Mrs Jackson (née Pamela Mitford) didn’t reply. Two for the book, two against it, then, and one on the sidelines.

Diaries of Unity’s that came into my hands by luck in Vienna provided the framework of dates, names and places. The lawyers sent a proof of the finished book to the Mosleys and the Duchess, and a campaign then followed to excuse Unity and discredit me, with just a hint of violence. The bulk of Unity’s papers are at Chatsworth, the great house in Derbyshire of the dukes of Devonshire. How did I know what I had written, and how had I obtained the photographs of Hitler and Unity reproduced in the book? The papers were where they should be, so I stood accused of breaking into Chatsworth once to take them and a second time to replace them. A friendly marchioness, no less, staying with Lord Lambton, rang me up late one night with the information that the house party condemned me as a traitor to my class. An ex-politician disgraced in a sex-and-drugs scandal, Lambton had been listening in to the marchioness on the tele-phone and in an incoherent review he indeed accused me of trying to show that the British upper classes were in his formula ‘responsible for inclining Hitler towards his Jewish atrocities’. John Gross, then editing the Times Literary Supplement, found the review he had commissioned from the socialite Alastair Forbes too malicious to print. Whereupon Forbes entitled it ‘The Piece the Jews Rejected’ and put copies in the post to everyone he could think of.

Disproving generalisations about the upper classes, the Duke of Devonshire let it be known that he did not share the duchess’s opinion. Unity had taken a political position in public, in his view, and comment was therefore in order. At a dinner in Chatsworth, I heard on the grapevine, the shouting was so fierce that the Duke had sent the servants from the room. Over the years whenever the Duchess and I might have met, she looked past me as though I wasn’t there. Perhaps that was not the intention, but on the pretext of loyalty to Unity, she and Lady Mosley were covering up for Nazism.

On the day war was declared, Unity shot herself in a Munich park, not fatally but leaving a permanent head wound. Returning home in early 1940, she secreted mementoes that surely would have landed her in prison if the authorities had discovered them. Her mother, Lady Redesdale, cared for her, most of the time on the lonely Scottish island of Inch Kenneth. When Unity died of her wound in 1948, Lady Redesdale presented these mementoes to a neighbour on the mainland who had shown pity. She passed them on and decades later they came to someone who — another stroke of luck — found me through the miracle of Google. This astonishing material, which has been authenticated, bears on her relationship with Hitler, shows how she had worked her way into the core of the Nazi hierarchy, and may explain why the Duchess of Devonshire and Lady Mosley were so anxious to hide everything under the carpet.

Inscribing a photograph of himself, Hitler had addressed Unity by her second given name of Valkyrie and by her nickname of Bobo. On the back of this photograph is the translation in an unknown hand of his German, ‘I am always with you however far away you may be. You are always next to me. I will never forget you.’ ‘Für meine Walküre Unity,’ he signed on the title page of a presentation copy of Mein Kampf. Somehow on that same page she had collected the signatures among others of Göring, Himmler, Hess, Speer, Ribbentrop, Goebbels and his wife Magda.

At a Nazi rally in 1935 she had praised the notorious anti-Semite Julius Streicher and then published her credo in his newspaper Der Stürmer, ‘Everyone should know that I am a Jew-hater.’ The translation in her hand from the German on the back of the photograph of himself that Streicher gave her reads, ‘You are beautiful. You are my Everything. My heart will always belong to you.’ A pile of visiting cards, 23 in all, evokes the life she had led. These men and women proved their friendship by writing their private telephone number on their card. Here is Hitler’s card with his private Munich number and the tip-off in his handwriting that in an emergency she should ring Julius Schaub, his adjutant. Erich Kempka always drove Hitler, and Unity boasts, ‘My Driver’ on his card. Leni Riefenstahl, Hitler’s favourite film director, asks her to ring soon. On the card of Magda Goebbels, the murderer of her children at the end of the war, Unity has written ‘Yum Yum!’ Eva Braun and her sister Margarete give the same address in Munich and Unity has inked a cross from corner to corner over both their cards. One of Hitler’s decisions in the final days of the war was to have Margarete’s husband, General Hermann Fegelein, shot for disloyalty, which is presumably why Unity smudged out the ‘Yum Yum’ she had awarded him. It speaks for itself that she had the cards of Reinhard Heydrich, a principal organiser of mass murder, and of Dr Karl Brandt, who was hanged by the Allies for torturing victims on the pretext of conducting medical experiments.

On a sheet of the Inch Kenneth writing paper, at some point in the war she wrote a sort of address to God. Whether it reflects the inner self or derangement after the failed suicide, at least Lady Redesdale thought it worth preserving and handing on. Unity praises some and blames others, among the latter ‘that traitor Mosley’. Whether he had betrayed England or Germany she does not specify. In her pre-war style, she prays to God to smite Hitler’s enemies, ‘especially the jews [sic] which will serve them all right’. As for herself, ‘Make sure that I go to heaven and sit there with the fuhrer [sic] for ever and ever.’