Throughout his life, Jeremy Thorpe had a knack of getting other people to do his dirty work for him

For years I’d wanted to write about the Thorpe affair – partly because of its extraordinary mix of scandal, deceit and astonishingly bad behaviour. And partly because of what it says about the Britain of almost half a century ago – a place where homosexuality was still taboo and where the Establishment regularly closed ranks to protect its own members. With all the stories that have come out recently, people might assume that there can’t be anything left to say about hypocrisy and cover-ups in the corridors of power. But the Thorpe affair easily trumps everything else in this regard.

What fascinated me most of all, though, was the fact that at its heart it’s a story about betrayal – about friendship gone horribly wrong. Throughout his life, Jeremy Thorpe had a knack of getting other people to do his dirty work for him. No one was more loyal in this regard than his fellow Liberal MP, Peter Bessell. But when Thorpe came to trial in 1979, Bessell was the Chief Prosecution Witness against him. I wanted to try and get to the bottom of what prompts someone to turn against his best friend.

However much I wanted to write about the Thorpe affair, I still faced what seemed to be an insoluble problem: I couldn’t work out how to tackle the story in a way that was fundamentally different to anything else that had been written about it. And then I had an extraordinary piece of luck. I happened to mention to a friend of mine that I wanted to do something on Thorpe. 'Oh,' he said, 'Do you want to meet Norman Scott?'

It turned out that his sister-in-law was living in the same village as Scott – Thorpe’s former boyfriend and the intended victim of his murder plot. Immediately, I threw some things into the back of my car and drove down to see him. Now 75, Scott’s memory of the whole affair turned out to be crystal-clear. Sitting in his living room, I switched on my tape-recorder – and all at once saw just how this book could be written.