Harry Thompson, Britain's most successful and brilliant TV producer, has written a novel. The novel is called 'This Thing Of Darkness'. It's a big chunky book with a ship on the front; the story is rangy and gripping; doubtless a bestseller is born.

That is the gist of the article which should have run on this page. If the events of the past two months had never taken place, it would have made a natural piece for a Sunday newspaper. Nice light read. There would have been some information about Harry Thompson's previous best-selling books (biographies of Peter Cook, Richard Ingrams and Hergé) and the various television series he has launched, including Have I Got News For You, They Think It's All Over and The 11 O'Clock Show. There would have been a merry interview with Harry being asked: 'Whither satire?', and 'What about this scandalous love life of yours?'

But Harry has lung cancer. A couple of months ago the doctors told him that his clingy flu, which was starting to look like pleurisy, was not pleurisy after all. Neither was it their second guess: tuberculosis. Harry's problem was, in fact, the two inoperable cancer tumours on his lung. He is 45, he plays sport all year and eats only organic food; he has never smoked a cigarette in his life. But lung cancer it is. Whither satire?

Harry is now in hospital, where he is being treated for pneumonia and an abscess on the lung. The primary cause is currently untreatable.

'I'm very irritated by the queue of diseases,' he says. 'When I first got the diagnosis - OK, I've got cancer, I'll deal with it. But it's very irritating to have a litre and a half of pus in my chest, and pneumonia, when I was just psyching myself up to go head-to-head with cancer. It's like a really big hard bastard has invited me outside the pub, and when I get there I find he's brought two of his mates who want a fight as well.'

Harry is in no position to turn up at radio stations right now, chatting about his new book between snatches of light music. And you can hardly send a troupe of pressmen into a cancer ward, tramping past the radiation suites in their trilby hats. The interviews which were due to attend the launch of this major novel have had to be cancelled.

But I was going to see him anyway. Harry Thompson is my ex-boyfriend. It was not a simple relationship: it caused pain and sadness and I'm not proud of that. It worked out about as well as we deserved, and I was hurt and angry with him myself for a long time. I haven't even seen him for a few years. When somebody gets cancer, though, all bets are off. Life's too short. As Harry says now: 'Why waste the utter preciousness by bickering or squabbling with anybody? I'm certainly going to be a lot nicer to people.'

So I went to see him and I ended up going back with a tape recorder. It seemed like the thing to do; I hope I'm not wrong about that. We have all moved on and Harry is happily settled with a woman whose loving strength has proved 'inexpressibly brilliant' in his time of need.

This novel is officially the first in a heavyweight two-book deal, and Harry is a man with a professional Midas touch (aside from the biographies, the 93 episodes of Have I Got News For You and 100 episodes of They Think It's All Over, he has produced The Harry Enfield Show, Newman and Baddiel and won awards for his latest satirical creation Monkey Dust). The book is good and it will be big and it deserves attention. Somewhere in here, I hope you will find the ghost of the piece which should have been.

This Thing Of Darkness is a historical tale about the 19th century voyages of the Beagle. The idea was born when Harry heard about a strange experiment by Captain Robert Fitzroy in the 1830s: selecting four Patagonian Indians he had met on his travels and bringing them back to England to be educated in a Walthamstow infant school. The clincher was Harry's discovery that Fitzroy was the captain of the Beagle. When the ship set sail to take the Patagonians home again, Charles Darwin was on board.

The result is one of those novels which people feel good about reading: an easy and picaresque yarn made muscular by historical fact and philosophical ideas. It was the natural first book for a man who had previously specialised in non-fiction.

'I was drawn to what we would now consider the perversity of the approaches,' says Harry. 'Fitzroy was a Christian Tory who fought to prove that black and white were equal. Darwin was a Liberal who believed in the workhouse and was horrified by non-white faces. Because Darwin is an admired figure, there is a conspiracy not to ascribe racist views to him. We see him as a kindly, white-bearded figure who freed us from the shackles of ignorance. Well he was fantastically racist and no they weren't all like that then.'

It is Fitzroy, rather than Darwin, who emerges as the book's hero. 'At its heart,' says Harry, 'it is the true story of someone who epitomised a certain sort of person that this country produced in the 19th century. There was a fantasy of chivalric empire, run by Britons who were gentlemen and played the game. Of course the reality was that our empire was no better than any other. We were busy conniving in the extermination of tribes, robbing natives of their land and we sent droves of brilliant young men, brought up with the chivalric fantasy, to enforce what was in many cases a visibly corrupt system.

'What usually happens is that young men become compromised as they get older: I imagine that Jack Straw, who can quite happily order 20,000 people to be bombed out of their houses to get some oil for the Americans, was once a student leftie with principles. But Fitzroy's morality was iron. He said no. And it destroyed him.'

And so, amid the sea storms and the colourful landscapes, the building of empire and the comic tale of the four Patagonians in the infant school, the two main characters debate morality, nationality, biology, fate and God. Crisis comes when Fitzroy's wife and Darwin's daughter die from incurable diseases, crushing both men and driving them to irreconcilable world views.

'This was the first really industrial generation,' Thompson explains. 'Fitzroy was determined that the world was a machine and everything in it worked to a pattern. He believed in God and the possibility of surveying the theological universe. So he has to believe that the random cruelty of a hideous disease is part of God's holy purpose. But Darwin finally denounced God when his favourite daughter died. If Annie could die in a revolting, unexpected, unfair way then there could not be a God. No, thought Darwin, we're all just monkeys and we're all going to die.'

It becomes impossible not to look back over these pages in the light of Harry's new condition. 'It is ironic,' he admits, 'that the vicious unfair lottery I wrote about should have arrived on my doorstep. But I wouldn't write it differently now.'

The novel implies that medical treatment has always been a stumbling, falsely confident endeavour. The writer has some fun at the expense of the 'modern European medicine' of 1830, which sees itself as superior to native practices. One chapter shows the scorn of the ship's doctor as a group of Patagonians attempt to cure a baby's fever by swinging a watch over its head, while he himself knows the true scientific cure: a glass of hot port.

'What those "modern doctors" of 1830 used,' says Harry, 'is the equivalent of Patagonian twigs today. We may laugh - but we don't know how to cure cancer, do we? 150 years from now, people will probably laugh at the thought of chemotherapy.'

Harry will have chemotherapy, though, as soon as he's well enough. He'll have everything. He'll have chemo and radio, health food supplements and 'pills that come in a screw-top bottle with a wavy fern on the front'. He's thinking about going to America to try the procedures there.

'Why not?' he says. 'There is no anti-cancer drug per se, so I'll do the fucking lot.' Ultimately, Harry believes in willpower. Even before he was ill, he had faith in the power of the mind to outwit disease.

'I'm going to beat this,' he says. 'I'm aware there's a technical possibility I might not, but I'm not countenancing that. It's the willpower that will get me through. British doctors won't talk about mental attitude; they assess you as a clinical case. American doctors are more like boxing coaches, urging you to win the fight. They're right to be. Some people get through on willpower and twigs alone. I'm not going to try that, but I know it's possible.'

John Diamond would have disagreed. That great and well-loved journalist, much missed since his death from throat cancer in 2001, devoted his final months to dismissing the idea of disease as 'a fight'. He hated the implicit corollary of the willpower argument: that those who fail to think about their illness in the right way are responsible for their own deaths.

But there are many responses to cancer and Harry will stand for only positive thinking. His chipper confidence is reassuring for those around him, and he feels sure it will be the key to his recovery. His own mother, he believes, was in control of the breast cancer which killed her in early middle age. Given only a few months' prognosis by her doctors, Harry's mother announced that she intended to live a lot longer than that; she died nearly 10 years later.

For as long as I have known him, and decades before his own inoperable tumours were discovered, Harry has considered that a demonstrable triumph of will. 'The problem is,' he says, 'I don't know how to do it for myself. It's quite difficult psychologically. I understand that I have to be strong-willed, but I can't decide whether I should be aggressive to the cancer or superior to it. Should I say 'Come here you little fucker, I'm going to kick you from one end of the room to another'? Or 'Oh God, you're so dull, just piss off while I get on with my life'? I haven't worked that one out yet.'

But he is getting on with his life, mentally at least. He is doing this interview. He is thinking about the second novel, and hoping to finish a 'little book about cricket' in the meantime. He has just agreed to write a sitcom. As far as the first novel is concerned, he is 'pathetically confident. If I get a bad review, I'll be flabbergasted. I think this is the best thing I have ever done.'

He may be right. There is an impressive body of work behind him, but this feels like a kind of zenith. He has been working towards pure narrative for years. His Cook and Ingrams biographies are structured like novels. The reason his panel games Have I Got News For You and They Think It's All Over were better than anyone else's is that they had a strong line of sitcom shoring them up from beneath. There were running jokes, familiar characters and storylines. His hosts and team captains related like dysfunctional families: Angus Deayton as the pompous father figure, Ian Hislop as the goody-goody son and Paul Merton as the brilliant, rebellious teenager.

If Harry had still been at the helm when Deayton got into trouble ('What balderdash that they fired him, as if you have to be morally competent to compere a quiz show'), the story could have run and run.

That is where Thompson always found his voice: in the gap between fact and fiction. He loved researching This Thing Of Darkness, poring over contemporary novels for slang and history books for fashion or transport, gathering the ingredients for a perfectly convincing tale where 'every time a character puts on his trousers or gets into a stagecoach, it's the right one'.

And he himself is like a character in a book; sometimes a book written by himself. Always a great story. Arguments with TV companies have become great epic battles of right and wrong. He doesn't go on holiday to France, he journeys across the globe to Namibia, Antarctica, Yemen and Timbuktu.

His amateur cricket team is called The Captain Scott XI: bringing to the pitch a romantic pre-written tale of failing with noble pride.

As for the 'love rat' tag which sometimes attends him, Harry is baffled by it. He can't understand why he would be gossiped about and called a Lothario when others have 'shagged more people'. My theory is that it's because the Harry stories are always better. It was never just a boring old shag. Always romance, heartbreak, moral complication and a range of conflicting viewpoints from everyone concerned. And when Harry tells you the story again, it is never quite the way you remember it. But it's always a Great Story.

I think he has found his vocation as a novelist. There it all is: the romance, the passion, the humour, the darkness, the perfect creation of a parallel world. In his TV shows, as in this historical novel, fact and fiction blend so neatly that one can't spot the join. The same has been true in his personal life (though perhaps not now, with Lisa, in a new and deeply intimate dynamic) and I have often suspected that he does not spot the join either. He was born to be a novelist. I'm glad he is already planning his second.

But he has another story to write first. The story of the man who was told he had lung cancer and overcame it by willpower. The abscess healed, chemo shrank the tumours, they became operable and Harry returned to a life of books and cricket and brilliant television. He was nicer to people. He never had an argument again. I am not looking for the join.

· 'This Thing Of Darkness' is published by Hodder on 20 June.

The fee for this article has been given to Cancer Research UK, Freepost Nat 12046, Sheffield S98 1BR.