It was enough to persuade James to invite Nixon to his home. "As a fiction writer, you've always got people coming up to you saying, 'I've got a story', and so I have learnt to listen, because you never know."

And so the following week, on his doorstep was an elderly man, plausible enough for James to agree to read the 1000-page manuscript he had brought.

Why, I wondered aloud, had Nixon picked out James? While today his name is well-known for the Roy Grace books, which have sold 19 million copies worldwide, back in 1989, he had, by his own admission, managed only four titles, of which the first three, all spy thrillers, had "tanked". The key to this particular mystery, seems to be the subject of James' fourth novel, Possession. "It was about a family who went to a medium after their son had been killed in a car crash. It had done very well."

The day after Nixon's visit, James was travelling to Bristol for a radio interview, and took the 1000-page manuscript. "After 20 pages, I'd lost the will to live. It was all ramblings from the Bible, tracts and annotations."

And there it might all have ended, had not the interviewer casually mentioned the site in Glastonbury where Nixon had told James the grail was buried. "It sent a shiver through me," he remembers. "I left feeling freaked, so I phoned an old friend, Dominic Walker, who was then the Bishop of Reading."

The bishop gave him three pieces of advice: that he needed more than three sets of compass coordinates to prove God exists; that the "something more" should be a miracle of faith beyond the laws of physics; and that, if he found it, there were enough church and political leaders with a stake in the status quo to have him assassinated.

It was, James, says, his "light-bulb moment". Between them, Nixon and the bishop had given him a sensational plot. The book idea continued to niggle, especially that central question of what would be required to prove God existed.

It has been an itch that James has been scratching ever since. There was a five-day pilgrimage to Mount Athos in Greece, among the holiest sites for Orthodox Christians plus regular visits to an enclosed Carthusian monastery near James' Sussex home. And conversations, too, with theologians as well as with what he refers to as "hardcore" atheists.

All this he has finally decanted into a fast-paced thriller. But the challenge was more than professional – it has clearly become a bit of an obsession for him.

"I've always been fascinated with why am I here, will there be something after I die, what is good, what is evil? So this book gave me a fantastic opportunity."

And in seeking out proof of God, has he ended up any more convinced? He pauses before answering, picking his words carefully. "Writing the book has given me a faith in informed intelligent design."

And it's thanks, in part, to Nixon and that phone call. The novel is dedicated to Nixon, who died five years ago .

The Daily Telegraph