You are aware that you have readers.” He had already finished his first novel, an ingenious children’s saga about miniature tribes living on a carpet called The Carpet People, but it took three years to find a publisher. Colin Smythe Publishing Ltd only sold 3,000 copies (they go for pounds 300 each now; the 1992 reprint made the adult bestseller lists), and Pratchett lived a part-time writer’s life during the Seventies, squeezing in a few hundred words a night. In 1980 he put his cheeriness to a new daytime purpose: soothing away anxieties about nuclear power stations as a press officer for the Central Electricity Generating Board. He “never knowingly lied”.PRATCHETT was rescued from this by a turtle-borne disc. In 1983 he finished a fourth and different novel, The Colour Of Magic, set, for the first time, on Discworld. The book was read out on Woman’s Hour and gradually, as he produced sequels, Discworld went from conceit to phenomenon. By 1987, “the success was going faster than my ability to comprehend it.” So Pratchett chose not to; instead, he hid in his cottage in the Mendips (“life was an eternal search for free wood”), and wrote faster.

The series acquired the following and gentle repetitions of a soap opera.But Pratchett had other purposes too. Like Hobbes or Swift, he made his invented world a paddock for his hobby horses. Some of these were banal: Moving Pictures feebly mocked Hollywood, which became a rainless and narcissistic studio town called Holy Wood. Others have been less obvious: in Feet Of Clay gibbets swing at every corner of Ankh-Morpork, but only wooden dummies hang beneath them, creaking by clockwork, faith in real executions as a deterrent having long passed away. Meanwhile the city itself, a republic sliced up and sustained by all-powerful Guilds (even an Assassins’ Guild, with rules about not killing bystanders), makes its own political inferences – even if Pratchett always denies them.He prefers to talk about his writing: every day, starting soon after eight, with a toilet roll beside him for the snuffling winter months. “When you have that original spark or idea the mental radio starts to tune itself,” he says, almost apologetically. Colin Smythe says: “He starts the next one on the afternoon he’s finished the last one.”But there are limits to Pratchett’s fecundity.

“I’m not saying that if I stopped all this I could be a ’serious’ writer,” he admits A planned detective series has so far proved abortive. And the Discworld books, for all their steadily rising credibility in the broadsheets, are still rarely taken seriously enough by literary editors to be reviewed by anyone other than Pratchett converts.This is all irrelevant to his readers, of course. What matters to them, besides the books, is the bubbling cauldron of Pratchett-chat, the Internet disputes about motive and plot, the rumours of new and lost works. “The cod idea arose that I had posted a chapter on the Net to someone, and I had lost my original copy… This idea persisted for four years.” Like followers of Star Trek, Discworld fans are not always content to let the writer decide the fates of their favourite characters.

Pratchett and Colin Smythe, now his agent, receive plot suggestions, proposals for books and, most of all, complaints in the rare event that protagonists die.Pratchett’s wife Lyn sifts his mail. He is careful to husband Discworld, only allowing film companies the rights to a few books and protagonists at a time. “He already has more money than he knows how to spend,” says Smythe. Recently Pratchett moved to a bigger house near Salisbury, with an outhouse for writing and a tennis court he doesn’t use.