Heard the one about the monk and the television satirist?

Even if you don’t know Tony Hendra’s name, you’ve probably laughed at his jokes: comic, scriptwriter, star of parody rock documentary This is Spinal Tap, and one of the brains behind Spitting Image.

Though he has lived in the US since the 1960s, an invisible cord has drawn him back again and again to a little Benedictine monastery at Quarr on the Isle of Wight, and to Father Joseph Warrilow, who had a word of counsel or consolation for every care in his often-troubled life.

Father Joe: The Man Who Saved My Soul is the story of their friendship. When it was released in the US last May, it rocketed to number five on the New York Times bestseller list. Hendra admits that he is baffled by its success. Though it is an unashamedly Catholic book, clear in its affections for the traditional forms of the faith, he has touched a far broader spiritual nerve. "We live in such a lonely world," he writes. "People just yearn for some kind of guidance and a hand to hold. I think it’s probably a relationship that people really do wish they had, and I’m extremely lucky to have had it."

Meeting Hendra in a London hotel, I expect him to be mouthy and brash with a mid-Atlantic accent and an arsenal of acerbic ripostes. Instead, he is all English reserve, by turns defensive and vulnerable, nervously fiddling with the top of his water bottle. Short and stocky with a full head of fair hair, he looks younger than his 62 years. His accent is entirely English. "In certain situations, I lose it very quickly," he grins. "Heavy traffic, for example."

He says his book is more a biography of Joe than a personal memoir.

In truth, it’s both. The story is Hendra’s and he tells it warts and all, the misguided blunderings of his youth, his failings as a husband and father, the drink, the drugs, the despair, and finally, the fresh start. But the presence of Father Joe pervades it, with his knobbly knees, lobster-pink hands and goofy grin. Perhaps part of its remarkable success lies in the fact that rarely has a friendship between two men been written with such unselfconscious affection.

The two met in 1956 when Hendra was 14 and caught in a "weird love triangle" with a young married woman who lived near his home in Hertfordshire. Catching them together, her husband, an ardent convert to Catholicism, marched young Hendra off to the monastery on the Isle of Wight. He expected punishment. He found Joe.

HENDRA WAS raised a Catholic by his mother, but until that moment his faith had little meaning. That day, as he listened to Gregorian chant for the first time while the Easter sunlight filtered into the chapel at Quarr, he responded at gut level to the possibility of faith. Returning from Quarr to life and school he committed himself to a life of faith.

He has little to say about his parents. Joe becomes his father figure, while his natural father, a stained-glass artist, seems troubled, distant and was absent fighting in the war during his son’s formative years. "I think they were a depressive parental generation," he says. "And we were rather an unhinged generation of children. We didn’t have that kind of parental bond. I think this is one of the reasons why Joe, this very unlikely father figure, nonetheless became my real father."

Hendra spent his adolescence preparing to enter the Benedictine order at Quarr, at least until his second year at Cambridge, when he had the second of his life’s epiphanies. It happened when he went to see Beyond the Fringe, the satirical revue written and performed by Peter Cook, Jonathan Miller, Dudley Moore and Alan Bennett for the Edinburgh Fringe in 1960. It was the first time he had encountered a power on a par with faith: it was laughter.

Soon he was in Cambridge Footlights, and in a comedy duo with future Monty Python member Graham Chapman: "We were very funny because, for the first and only time in my career, I was the straight man." Shortly after graduating, he married Judith Christmas, and they had the first of two daughters. They moved to the US, away from teenage monasticism, away from Quarr, away from Joe.

He remembers getting a letter from John Cleese in 1968: "‘We’re putting together this group. We’re thinking of doing a new show, and you might want to come back to England to talk about it.’ At the time, I had just been signed to NBC to do a series, and so I didn’t." The series never came to pass, but Monty Python did. He laughs. "I don’t think I was funny enough to be in Monty Python, but it makes a good story."

He started working for National Lampoon in 1970, delighted to be a scourge of the Nixon administration. It was, he says, like "having a second shot at being a teenager", his first having been spent abstaining from most things except the Benedictine Rule. It was less fun though, he admits, for his family. He writes that he largely resented his daughters "because I had to be a father, which prevented me from f****** as many hippie chicks as everyone else appeared to be."

LAST SUMMER, while Father Joe scaled the bestseller lists, he was shaken by allegations made by the younger of his two daughters from his first marriage, Jessica, now 39, that he sexually abused her. The New York Times published a long investigation into her claims. Hendra refuses to comment other than to say "for the record", that the allegations are "absolutely untrue".

In the early 1980s - with the Lampoon years behind him and his marriage in ruins - alone in a hotel in Santa Monica, he overdosed on valium and vodka. It was "existential, despairing... the bottom dropped out of everything", the lowest point in a life of dramatic trajectories. When he awoke the next day with nothing more than a thundering headache and went straight on to the set of This is Spinal Tap where he was playing manager Ian Faith, he felt that, once again, he had been saved. Though not the most faithful correspondent, his emotional link with Quarr never really diminished, nor did Father Joe’s steady flow of non-judgmental acceptance and sage advice.

When he met his second wife Carla, it was on Joe’s encouragement that he married her. Together they travelled to Quarr with their baby son, Nicholas, so Joe, then approaching 80, could hold his "grandson". The second of their three children was with Hendra on his last visit in 1998 when he discovered, with shock, that Joe, then nearly 90, was dying of cancer.

"He was one of those people you completely take for granted," he says. "You just assume that, however old they are and however infirm, that they’re going to be around forever. And when they do finally depart, it leaves a much greater void, because it is unforeseen. It was terrible. Everything just seemed to fall out of the universe at that point."

All the more so because Joe had played such an integral part in his faith. It was Joe’s God he was enchanted by the possibility of knowing: "gentle, generous, endlessly creative." Could he know that God without Joe? "I’m still struggling with that seven years later. I don’t understand, in fact, how people believe without somebody like Joe. I find it hard to understand how people respond to the Christ of the New Testament. It intrigues me, I’m jealous of it, if anything. I’m a bit baffled at how people arrive at a perception of God."

But in the void, a seed was germinating: a book. "For the first time, when I was writing the book, I found myself feeling a sense of active faith," he says. "I felt the presence of Joe, nothing spooky, but a real sense that there was something still alive somewhere that was compelling things forward."

I ask if he expects to meet Joe again. He says, quietly: "I certainly hope so, I think about it all the time, actually."

After Joe’s death, he got the biggest surprise of his life. He was not Joe’s only confidant. The old monk had dozens of penitents, visitors and correspondents, including Princess Diana and the current Archbishop of Canterbury, Rowan Williams. "It was an odd moment," he smiles, wryly. "But also, when I reflected on it, a very inspiring moment. I can’t imagine any spiritual mentor who wouldn’t mention to any of his spiritual mentees that his experience and trust was ratified by other penitents. Though I must say, I did think: ‘That old bastard, he was seeing other people!’"

HENDRA IS FOLLOWING his spiritual memoir with another surprise change of direction: a satirical novel about the religious Right in the US. Although in Father Joe he explores his doubts about the value of satire he believes it has a role to play.

"I do think certain people, certain groups, perhaps benefit from being portrayed satirically," he says. "In the sense that people don’t look at them in the same way again, and they shouldn’t. This is not going to be a hilariously funny book, it won’t set out to destroy people or make people look absurd. On the other hand, these are bad people, doing bad things. They’re the kind of people that think the world is coming to an end, and when Christ returns, he will murder millions of Muslims or film producers or stem-cell researchers, or whoever else they’ve got in their sights this week. That seems to me to be a worthy target for satire."

He and Carla considered moving to France when Bush was "not-elected" in 2000. They still have a house in the Pyrnes. "I may have to leave," he says, a hint of a smile playing on his lips. "When I write this book, I could be out on my butt." Which would prove beyond doubt that the serpent of satire still has teeth, and that would give him pleasure.

Father Joe by Tony Hendra is published by Hamish Hamilton, price 16.99