Anyone who knows anything about the Jeremy Thorpe Affair can instantly reel off the name of the gunman who shot Norman Scott’s Great Dane Rinka: Andrew “Gino” Newton. Similarly, Thorpe nuts — an erudite if excitable bunch — are unlikely to be troubled by Newton’s all-too-apt nickname: Chickenbrain. But what’s less well known is that before Newton embarked on his ill-starred trip to Dunstable to shoot Norman Scott (who actually lived in Barnstable), another would-be hitman was offered the job.

This was a dealer in antique firearms called Dennis Meighan who claims to have been approached by a man describing himself as “a representative of Jeremy Thorpe”. Would Meighan consider “silencing” Scott for the good of the Liberal Party, and a £13,500 fee? Meighan was sufficiently interested to drive down to Devon to have a look at Scott before getting cold feet and heading back.

Later on, he had a sudden, if inexplicable, attack of conscience and confessed to the police. A few days later, Meighan was asked to come into a West London police station to sign his statement. But once there, he was surprised to find that all references to Thorpe and the Liberal Party had been mysteriously deleted.

The veteran BBC reporter Tom Mangold, who made the BBC4 documentary The Jeremy Thorpe Scandal, is convinced that the order to “doctor” Meighan’s statement must have come from very high up indeed, possibly from the then Home Secretary. If so, the finger would point at either Robert Carr or Roy Jenkins — Meighan is a bit vague about dates.

I mention this just in case any readers are inclined to dismiss Adam Macqueen’s debut novel as being too far-fetched. With great ingenuity, Macqueen has taken the bone bones of the Thorpe Affair, done some judicious tweaking, and turned them into a first-rate conspiracy thriller. In the Macqueen version, a former rent boy who goes by the name of “Tommy Wildeblood” investigates the murder of another “renter” whose body is found in Hampstead Ponds — Peter Wildeblood, you will doubtless recall, was one of the last men to be imprisoned for homosexuality in England.

It’s an investigation that leads Wildeblood from the public toilets of Soho up to the highest levels of government, taking in Tom Driberg, Cyril Smith and Harold Wilson’s “Spymaster-General” George Wigg along the way — as well as Marcia Falkender and her famous handbag, whose contents, she always claimed, could cause Wilson’s instant ruin.

One of the remarkable things about Macqueen’s book is how little tweaking he has had to do to make his narrative plausible; if anything, his alternative reality often makes more sense than the real thing. In Macqueen’s version, it wasn’t Rinka who was bumped off on that stormy night on Exmoor in October 1975. Instead, it was “poor old Norman” who copped a bullet through the head.

To cling on to power, Wilson desperately needs the support of the Liberals, led of course by Jeremy Thorpe — in reality, it was Ted Heath who toyed with the idea of a coalition, but eventually talks came to nothing.

All this raises an intriguing question: what do you do when you’re prime minister and your coalition partner turns out to be a homicidal maniac? As portrayed here, the answer is simple: you use every means at your disposal to stop his murderous escapades from becoming public knowledge.

Despite only being one year old at the time, Macqueen has caught the spirit of the times perfectly. This is a world where Emmanuelle II (directed by the — once again — all-too-aptly named Just Jaekin), is showing in the cinemas; where the army surplus store Lawrence Corner near Euston is considered to be the height of shabby-chic; and where racists are still known as “racialists”.

But it’s not only the background details that are spot-on. Macqueen has also managed to make sense of one of the great mysteries of twentieth-century British politics: why did Harold Wilson suddenly resign in March 1976? Was it because he realised he was suffering from the early symptoms of dementia (the official version)? Or was it because he knew there was every chance he would end up in jail if he didn’t (Macqueen’s)? And just in case you should be wondering why the right-wing tycoon, James Goldsmith, did his damnedest to get Private Eye closed down back in those dark, demented days, there’s an eminently satisfying explanation to be found here.

Best of all, Macqueen has managed to create a really memorable main character. Like his namesake Peter, Tommy Wildeblood is brave, clever and brimming with moral indignation. But he’s also vulnerable, wanton and willing to do just about anything to keep his head above water.

After I finished writing A Very English Scandal, I took a solemn vow — that I would rather spit-roast my own offspring than read anything else about the Jeremy Thorpe Affair. Seldom have I gone back on my word with more pleasure. As boldly conceived as it is vividly realised, Beneath the Streets is a delight.