More than twenty years have passed since Michael Bloch interviewed me for this biography of Jeremy Thorpe. The book’s gestation period has been ten times longer than an elephant’s.

The public life of its subject, by contrast, whizzed by like that of an Edwardian-suited mayfly. President of the Oxford Union at twenty-one, an MP at thirty, Liberal leader at thirty-seven, he was effectively offered the deputy prime ministership at the age of forty-four. He might have taken it, too, had not his fellow Liberal MPs been understandably revolted by the idea of getting into bed with Ted Heath. In May 1979, five days after his fiftieth birthday, Thorpe lost his seat in Parliament. Five days after that he was in the dock at the Old Bailey, standing trial for conspiracy to murder Norman Scott, a former male model.

And that was that. Despite his acquittal he became an unemployable pariah and public sightings became ever scarcer as Parkinson’s disease took hold. In a biography of more than six hundred pages, the last thirty-five years of Thorpe’s life are dispatched in just thirteen. Still, now we know why the book took so long to appear: when Bloch showed Thorpe the draft manuscript in 2001, ‘his reaction was confined to an urgent insistence that it should not appear in his lifetime’. He died last December, aged eighty-five.

There are plenty of tales here that would have made Thorpe wince, none more surprising than the suggestion that he may have ordered another murder, that of his former patron the Hon Henry Upton, who disappeared off the Sussex coast in 1957. ‘High-speed motorcycling was one of his twin passions: the other was a compulsive, sadistic homosexuality,’ Bloch writes of Upton, in one of the startling formulations that occasionally leap off the page like a frog from a sherry trifle. My favourite is on Jeremy’s father, John ‘Thorpey’ Thorpe, a barrister and politician: ‘Like many sensitive men who suffer from precarious health, he was in his element at funerals and memorial services.’

Thorpey died when Jeremy was fifteen. From then on, and for many decades afterwards, the boy was in thrall to his fearsome, monocle-wearing mother, Ursula, presented by Bloch as a cross between Lady Bracknell and Bertie Wooster’s Aunt Agatha. It had been a severe disappointment to her when Thorpey lost his seat after only one term, so hopes of glory now rested on her son.

She of all people should have known that the paths of glory lead but to the grave. Her father, the colonial entrepreneur ‘Empire Jack’ Griffiths, had also served as an MP when not building railways in Angola or aqueducts in the Caucasus. Like his grandson Jeremy, he was a compulsive self-publicist and risk-taker. Facing ruin and possible imprisonment when his business collapsed in 1930, he shot himself. Thorpe may have had this in mind when, hours after becoming leader of the Liberal Party, he promised his friend Peter Bessell MP that if his gay affairs became public, ‘I shall blow my brains out’.

Whether or not he meant it, this was a typically melodramatic pose. As Bloch emphasises, recognising the importance of play-acting and stagecraft is the essential prerequisite to understanding Thorpe’s life. At Oxford in the late 1940s, with clothes still rationed, he had a full wardrobe of frock coats and brocade waistcoats, accessorised with silver-topped canes and a brown bowler hat. From early childhood he was a brilliant mimic, ‘imitating not just the voices but the expressions and mannerisms of his subjects’.

It isn’t too fanciful to see the public persona of Jeremy Thorpe as his greatest role, one he played with total commitment and without ever losing sight of the audience. His marriage to Caroline Allpass in 1968 may have become a genuine love match, but it was also a performance, intended to deflect rumours about his sex life. After the birth in April 1969 of their son, Rupert, he became a truly doting dad, but also one who saw the dramatic potential of fatherhood, enjoying ‘fanfares of publicity’ when he had Rupert christened by the Archbishop of Canterbury.

Caroline died in a car crash a year later. Thorpe’s grief was undoubtedly real, but Bloch notes that he played the bereaved husband ‘with characteristic theatricality’. He erected a huge monument to her in Devon and organised a full-dress memorial service in London, attended by most of the new Cabinet, with an address by the archbishop and a performance by Yehudi Menuhin.

A former lover complained that he was a ‘ham actor’, but what choice did he have? Leading a double life was obligatory at a time when homosexuality was a criminal offence – and especially in the 1950s, when the defection of Guy Burgess to Moscow in 1951 made it officially synonymous with treachery. The home secretary, Sir David Maxwell Fyfe, promised a ‘new drive against male vice’ that would ‘rid England of this plague’. Lord Montagu, whom Thorpe had known at Eton and Oxford, was jailed in 1954 for ‘conspiracy to incite certain male persons to commit serious offences with male persons’.

From Thorpe’s first encounter in 1960 with the twenty-year-old Norman Scott (‘looking simply heaven’), this book becomes the chronicle of a doom foretold. After a brief sexual fling, Scott haunted Thorpe for almost two decades. He would ask for money to start a new life; Jeremy paid. Then he would fall out with his new employer and come trotting back to Westminster, demanding another ‘retainer’ to stop him blabbing. Eventually he started to blab anyway, despite the hush money. ‘We’ve got to get rid of him,’ Thorpe told Bessell one day. ‘It’s no worse than shooting a sick dog.’ Thus began the plot that landed Thorpe in the dock at the Old Bailey.

Did he mean to kill Scott, or merely to give him a fright? Either way, it’s a preposterous yarn, expertly narrated by Bloch, in which the best man from Thorpe’s wedding consults a Welsh carpet merchant, who recruits a Swansea nightclub owner, who hires an airline pilot and part-time hit man named Andrew ‘Gino’ Newton, who shoots Scott’s Great Dane, Rinka – as a consequence of which Thorpe is accused of conspiracy to murder and the late editor of Literary Review, Auberon Waugh, stands for Parliament against him on behalf of the Dog Lovers’ Party.

At which point you may well scratch your ear and think: can any of this be true? It reads like a shaggy-dog story, fittingly enough, and credulity is stretched even further by some of Bloch’s supporting cast: DCS Proven Sharpe, head of Devon and Cornwall CID; his chief constable, Colonel Ranulph ‘Streaky’ Bacon; Norman Scott’s most sympathetic landlady, Mrs Friendship; his kindly Jesuit mentor, Father Sweetman. And then there’s the handsome young Buckingham Palace footman who had an affair with Thorpe and wrote about it many years later in his memoir, Adventures of a Gentleman’s Gentleman. His name? Guy Hunting.