‘Do I,” says Terry Pratchett suddenly, “have a small yellow moustache at the moment?” He does. His wife Lyn has just given him a turmeric drink.

“There is some evidence from America that it has some effect on Alzheimer’s, slows it down, but anyway I like it. She had me on that from the word go.”

The “word go” was spoken on December 11 last year when Pratchett told the world - a great deal of which reads his books - that he had Alzheimer’s. It was early onset; he’s only 60.

It was a globally significant moment. Pratchett has sold 60m books. He writes an average of two a year and he earns £1m from each. Discworld, in which almost all his novels are set, has become an alternative reality for Pratchett fans around the world. Millions are bereft, many sending him “cures”.

We meet in his “chapel”, a cluttered studio in the grounds of his Wiltshire house. It’s stuffed with technology minded by his assistant Rob. In one room he works on an array of five computer screens. The other room is a seemingly chaotic library.

“We’ve had mice,” he says, apologising for the mess, though I suspect it’s usually like this. Small, black clad with a startling domed head, he addresses me from a grand throne-like chair next to a preposterously elaborate lectern.

He is universally liked for his easy, entertaining style. Announcing his diagnosis on the internet, he called it an “embuggerance”, though he affects to be dismayed by his own good humour.

“The day after I had been diagnosed I was working in the garden and I suddenly realised I was whistling, and I thought regretta-bly there is this sort of inner well of humour or good nature, there is some kind of insuppressible source of good humour that I can’t actually manage to get rid of.”

He was genuinely angered, however, to find that he and others of his age are too young to get the Alzheimer’s drug Aricept on the NHS.

“If I ate myself into obesity I could get pills for that for nothing. If I wanted Viagra I could get that for nothing. But I can’t get a drug that gives me that little bit of extra edge. I can afford £90 a month, of course, but there may be someone who can’t in his fifties with early-onset Alzheimer’s with dependants - anything that gives an extra edge must be worth it.”

Last week’s Sunday Times story that patients who paid for their own cancer drugs would be denied NHS treatment enraged him. “In the early days of the NHS, if someone had a bit of spare cash they would hand it over to their doctor and he’d say thank you very much. I cannot see how paying for their own drugs undermines the NHS.”

Feeling that the disease is ignored, he has given more than £500,000 to Alzheimer’s research.

Anger aside, Aricept and/or turmeric seems to be working for him. His condition has improved since December. In the car, he no longer has to keep stabbing away with the seatbelt; he can fasten it in one. Dressing, he’s no longer baffled by his clothes; he just puts them on.

Aricept means he can’t drink, but he’s taken up snuff “because it has an interesting historical background; it’s made of ground-up churchwardens, you know”. The fact that it might harm him is a perverse consolation.

“I take the view that it may be bad for me in the long term. On the other hand, if it is bad for me that is because I havea long term, and from where I am sitting, the long term seems like a very good idea. This may be the time to take up free-fall parachuting.”

Bad typing - the first sign that something was wrong - remains a problem. His productivity is affected. He’s easing down from two to one book a year. The danger is that the embuggerance will be seen in everything he writes. Although he’s probably had Alzheimer’s for three years, however, his last book, Making Money, was well up to scratch; and Rob says his next, Nation, due in the autumn, is first-rate.

Anyway, to the point of our meeting: Discworld is 25 years old this year. “Good heavens!” he cries, relieved that we can get off the subject of Alzheimer’s, “Yes, we can actually talk about me as a writer!”

In 1983 he published The Colour of Magic, set in a disc-shaped world that sailed through the universe supported by four elephants standing on a turtle. Discworld rapidly annexed the real world and he was soon making millions. He was the biggest selling British author in 1996. Then JK Rowling overtook him.

Ah, JK Rowling. He got into trouble a few years ago for mocking her claim that she did not write fantasy. “I’m not the world’s greatest expert,” he said, “but I would have thought the wizards, witches, trolls . . . would have given her a clue.” Hate mail ensued.

So now when I bring up Rowling he sits there comically tight-lipped. I get round this by talking about the novelist Margaret Atwood, who displayed similar genre snotti-ness when she said that Pratchett didn’t write sci-fi but “speculative fiction”.

“Oh good! Right!” he roars, “Well, I’m writing advanced folklore, perhaps – alternative folklore!” He slips into a prissy Atwood persona – “I’m just speculating about the future. It’s got robots in it, but it’s not science fiction.”

He hates the kind of genre apartheid that sends sci-fi and fantasy to some dank bookshop location – “like the VD clinic”. In fact the genres include great writers, not least GK Chesterton, one of his heroes (and mine). Fantasy is also a term too easily applied.

“There are mainstream novels that have more fantasy than some of mine . . . The point is that any fantasy in a book will turn it into a fantasy, whereas a murder in a book will not turn it into a murder mystery. I’ve written police procedurals, romances and murder mysteries; but because the person murdered is a dwarf, it becomes a fantasy.”

Equally irritating to him is repeatedly being asked if Rowling stole something from him – or he from her.

“Magical schools or universities, elves, trolls, orcs, unicorns – they’re low-hanging fruit. These are cultural things: nobody owns them and everyone is allowed to take them down off the peg and paint them a different colour.

“I would never have been able to write Discworld without the people ahead of me. Everyone is standing on the shoulders of giants . . .”

He embraced fantasy at the tail end of the largely Tolkien-inspired Sixties’ and Seventies’ fascination with trolls and elves. Then fantasy took off again in the late 1980s.

“It really dawned on publishers very late in the day that there was a fantasy boom going on. I just happened to be there at the right time when the wave came. That was in the late 1980s; then in the early 1990s the wave withdrew a little bit and publishers started looking at their lists.

“I was high enough above the line to survive. There’s another boom right now and one of the nice things about it is that a lot of people who were out of print are now back in print.”

And here’s a thing: Pratchett may have found God. He says he is an atheist; but after his diagnosis, something happened. At the time, he was busy with all the new demands on his time and feeling that, perhaps, he ought to be writing.

“I’m certainly not a man of faith, but as I was rushing down the stairs one day . . . it was very strange. And I say this reluctantly, because I am trying to deal with this situation in as hardheaded a way as I can. I suddenly knew that everything was okay, that what I was doing was right and I didn’t know why.

“It was a thought that all the right things are happening in the circumstances; and I thought, well, that’s all right then.

“I don’t actually believe in anyone who could have put that in my head - unless it was my dad, and he’s been dead a few years. Or maybe it was just me.”

It was his first such experience. Did it make him rethink his lack of faith?

“Faith in what? If I get pushed in this corner, I believe in the same God that Einstein did. Einstein was a clever bloke . . . And it is just possible that once you have got past all the gods that we have created with big beards and many human traits, just beyond all that on the other side of physics, there just may be the ordered structure from which everything flows.

“That is both a kind of philosophy and totally useless - it doesn’t take you anywhere. But it fills a hole.”

To mark the 25th anniversary of Discworld, Terry Pratchett will be signing copies of his books at Foyles, Southbank Centre, London, on June 14 at noon

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