Many still view Harold Macmillan as a great actor, with the ability to hide behind a variety of masks. When Supermac’s rival and colleague Rab Butler learnt that historian Richard Thorpe had been to see the elderly former Prime Minister, Butler waspishly inquired: “How was Harold Macmillan when you met him? Was he the duke’s son-in-law or the crofter’s great-grandson?”

Macmillan, true to form, had, of course, played both parts and many more.

Thirty-five years on, Mr Thorpe, who lives in Banbury, can still vividly recall that first meeting with the man he has spent more than three decades pursuing.

“It was on St George’s Day 1975. I had travelled to Birch Grove, Macmillan’s family home, aged 32 and had never written a book. But he agreed to talk to me about Sir Austen Chamberlain, Lord Curzon and Rab Butler.”

Mr Thorpe, now 67, can readily quote memorable phrases that Macmillan effortlessly summoned during an afternoon of fascinating reminiscences.

But it was the tears in the old man’s eyes that touched him most deeply. They came as they walked in the park where President John Kennedy’s helicopter had landed and taken off in June 1963.

The 81-year-old Macmillan paused to describe how it had “sailed down the valley above the heavily laden, lush foliage of oaks and beech”, before adding: “That was the great thing about Jack. He could always see beyond the trees.”

As Thorpe departed, he reflected on how he wanted to write a life of Harold Macmillan.

After writing numerous major works, including biographies of Prime Ministers Anthony Eden and Sir Alec Douglas-Hume, he has finally produced that book — Supermac: The Life of Harold Macmillan.

Mr Thorpe, a senior member of Brasenose College, Oxford, and a fellow of St Antony’s College, can claim to have interviewed just about everyone closely involved with Macmillan’s career, including all the Prime Ministers from Macmillan to Thatcher, ten Foreign Secretaries, ten Chancellors of the Exchequer and eight Foreign Secretaries.

Happily, he did not have to travel far to interview many valuable sources on Macmillan’s time as chancellor of Oxford University.

He was elected to the position in 1960 while still serving at Downing Street, having ignored pleas from colleagues, like Lord Kilmuir, who had warned him: “You risk your neck for something you don’t really need.”

Macmillan simply replied: “You might say the same thing about fox-hunting.”

Mr Thorpe’s book shows beyond doubt that it was the city that Lord Stockton (as he became) most loved, having first arrived at the age of nine, travelling in a horse-bus from the railway station to Summer Fields, the North Oxford prep school where he boarded.

Macmillan’s grandson, David Faber, who provided important new insights, has, in fact, just become head of the school.

Like his brother, Dan, Macmillan went on to win a scholarship to Balliol, his time at Oxford University being cut short by the First World War, allowing Macmillan to insist thereafter that he was “sent down by the Kaiser”.

Mr Thorpe, who taught history at Charterhouse for over 30 years, was educated at Fettes College in Edinburgh, leaving before Tony Blair’s arrival.

He recalls that his mother went to school in Huddersfield with Harold Wilson, whose biography Mr Thorpe came close to writing.

He prides himself in ensuring that Huddersfield, his home town, makes it into the index of all his books. In the new book, there is a reference to the 1938 Preston and Huddersfield FA Cup final, the first to be televised live.

He believes that his new book nails three of the great controversies surrounding Supermac.

The suggestion that Macmillan was thrown out of Eton for homosexuality is entirely false, Mr Thorpe suggests.

Macmillan was not expelled, the likely reason for his departure being that his dominant mother, Nellie, got wind that he was a potential victim of predatory older boys. Her three great fears for her young son, apparently, were homosexuality, Roman Catholicism and alcohol.

“There is some suggestion that he might have been emotionally damaged by his experiences at school, but no evidence at any time in his life to point to homosexual or bisexual activities,” concludes Mr Thorpe.

Sex is at the centre of another painful episode that arguably helped propel Supermac to the top of the political tree. His wife’s infidelity with the Conservative MP Robert Boothby was to cause Macmillan untold agony and humiliation.

We now know the full extent of the torment Macmillan endured over 45 years about who was the real father of his daughter, Sarah. Thanks to the discovery of a remarkable tape recording, Mr Thorpe believes we now have the answer.

In 1975 Macmillan went to a lunch at which Boothby was also a guest. He told his former love rival that he wanted peace of mind by knowing whether Sarah was his daughter.

Remarkably, that conversation between the two men was accidentally recorded and the tape remains in the possession of Boothby’s widow. Boothby had been recording a Tchaikovsky symphony on the radio, when Macmillan arrived at his flat, and turning off the radio, he unwittingly left the recorder running.

Boothby informed the old man that he could not have been Sarah’s father.

“Boothby was a rakish figure,” Mr Thorpe writes. “But in one thing he was very careful. Despite, or because of, having so many affairs, he ensured he never left behind what Victorians would have called a vestige.”

The third, and most important, myth laid to rest is the suggestion that Harold Macmillan was a mass murderer because of his role in the repatriation of Cossacks and Yugoslavs at the end of the Second World War.

The complex issue, involving missing files and a famous libel action (claims made on the episode by the Southmoor historian Count Nikolai Tolstoy resulted in Tolstoy being ordered to pay a record-breaking £1.5m damages to Tory peer Lord Aldington), is expertly unravelled.

Mr Thorpe concludes that Macmillan was no war criminal, rather “the messenger bringing unpalatable instructions”.

A signed copy of Supermac: The Life of Harold Macmillan has already been sent to David Cameron, whose former Oxford tutor Vernon Bogdanor hailed it as: “The best biography of a post-war British Prime Minister yet written.”

Mr Thorpe believes Macmillan has much in common with the present occupant of 10 Downing Street.

“Cameron sanitised the Conservative Party and made it electable. Macmillan took the Tory party out of the shadow of Churchill, the 1930s and unemployment. I believe Macmillan is the Prime Minister that Cameron holds up as his mentor. There is a picture of Macmillan in his study.”

And the differences?

“Well, Macmillan upped the ante when it came to Summer Fields and Balliol. He delighted in drawing attention to his status as the most famous living Old Etonian. He came from a famous publishing family but people in the Conservatives looked down on him because ‘he was in trade’.

“David Cameron is undoubtedly an upper class figure but you would not see him dead in an Eton tie.”