The life of an Oxford don might not sound, on paper, like the basis for an entertaining, incident-packed memoir, but Hugh Trevor-Roper was no ordinary academic. His biographer, Adam Sisman, explains: “He was completely exceptional.

“He was involved in the spy world, he was a major figure in journalism and politics. He was a star in Oxford. His students knew they were being taught by someone very famous: how many academics, after all, have a large Bentley parked in front quad?”

The son of a Northumberland doctor, he rose to become Regius Professor of History, earning a reputation as a formidable thinker and savage critic of other historians. Margaret Thatcher eventually made him Lord Dacre, but he first made his name interrogating senior Nazis for MI6 just after the Second World War. His subsequent book, The Last Days of Hitler, made him a global authority on Hitler; but it was in this role that he also took his greatest tumble. In 1983, he authenticated the so-called ‘Hitler Diaries’ for The Times newspaper. They turned out to be a hoax and his reputation never recovered.

His pugilistic personality makes for an engaging biography. “He was one of the spikiest people of the 20th century. He was notorious for it. His attack of Lawrence Stone [over the causes of English Civil War] still sends a shiver down the spine of most academics. He drove him out of Oxford. And he destroyed the reputation of [social campaigner] Arnold Toynbee.”

Trevor-Roper once said: “There is nothing so exhilarating as a good battle.” He tussled acrimoniously with fellow history don AJP Taylor and offended Evelyn Waugh, so that Waugh said that the only honourable course open to Trevor-Roper was to “change his name and seek a livelihood at Cambridge”.

Sisman has written several biographies, including one of Trevor-Roper’s great rival AJP Taylor, but this is the first where he actually knew his subject. He first met him in Oxford in the 1970s at a drinks party, also attended by Isaiah Berlin. Later, after Trevor Roper agreed to Sisman being his biographer, he knew him as an older man. What was he like?

“When I first met him, I was a very young junior editor at Oxford University Press. He was nice to young people, tolerant of their gaucheness. He picked on people his own size, rather than bullying small fry.

“I really got to know him in the 1990s. By then, he had become a much more benign figure, very friendly. Therefore I find it hard to recognise the figure who emerges from his letters and from earlier decades.”

Sisman was granted exclusive access to Trevor-Roper’s vast archive and enjoyed poring over it, though he finds it strange that he now knows the historian “better in death than in life”. What did he unearth that most altered his opinion of the late don?

“One startling thing was the discovery of all the love letters between him and the married woman he was having an affair with, Lady Alexandria Howard-Johnston; actually, some of the letters were discovered in a secret compartment in a desk, which was a very exciting moment. But what was surprising was how intense and passionate the letters were. Very moving. He was someone who was naturally very reserved. Isaiah Berlin described him as being 'all glass and steel’.”

Although he never produced a great historical work to cement his standing for posterity, he had a prestigious academic career. In his 70th year, however, he tarnished his reputation by authenticating the ‘Hitler Diaries’ — a hoax by German conman Konrad Kujau — for The Times. Why did a naturally sceptical man of such erudition fall for such a scam? Sisman argues that he was in a difficult position.

“He was fighting a series of battles with Rupert Murdoch, who thought him a fusty establishment figure. There was a pressure not to be so cautious. And he was told things that were not true, such as that the paper and handwriting had been authenticated.”

Sisman, who never shirks from highlighting the man’s many failings, as well as triumphs, concedes that it was “a terrible blunder and he shouldn’t have made it” — although he says that The Times headline on his death — “Hitler Diary Hoax Man Dies at 89” — was exceedingly cruel.

“As human beings, we can understand why people make such mistakes. He’s not the only learned man to make such a mistake. The great Dr Johnson was hoodwinked by a forger and David Hume, the great sceptic, was also fooled. We don’t judge them by their mistakes, and we shouldn’t judge Hugh Trevor-Roper by his.”

l Adam Sisman will be at the Woodstock Literary Festival on Sunday, September 19.