Prolific novelist Sir Terry Pratchett

“Would you have known I had Alzheimer’s Disease unless you’d been told?” I answer truthfully: “No,” but I think it was a rhetorical question. He barely paused for breath before he’s off again, talking 19 to the dozen, any question prompting a torrent of considered, articulate, well-informed opinion peppered with facts and jokes and stream-of consciousness digressions that usually, eventually, relate in some form to the original question. As I discover when we meet for lunch, my goodness, Terry Pratchett can’t half talk. My goodness, Terry Pratchett can’t half sell books, too. His new novel, Snuff, the latest in the Discworld fantasy series, is his 50th and the biggest-selling fiction hardback of the year (also the UK’s fastest-selling hardback in history).

To date, he has sold about 65 million books. If you were looking for signs of Alzheimer’s, (or, to be specific, his particular form of the disease: posterior cortical atrophy, which he has described as sitting “on top of Alzheimer’s”) you would be hard pushed to find them. On one occasion, he forgets who wrote a book he liked. Talking about Nation, his moral story for children, he says the book dragged him through a field of “tissues” then he pauses until the word “thistles” comes to him (and he blames the blip on the large brandy he’s enjoying over our lunch).

Which means he’s no different to anyone who hasn’t been diagnosed with Alzheimer’s. Now 63, he was only 59 when he was diagnosed but thus far, at least, he feels the disease hasn’t had a great impact on most aspects of his day-to-day life. “I know I’ve got it because I’ve seen the scans,” he concedes. Otherwise, because the disease means he doesn’t always see what’s before him, the main irritant is that he can no longer drive. “I was having some difficulties driving. It was just more stressful and made me wonder what was going on so it was a big relief to get shot of it,” he says philosophically. After all, he has his wife Lyn and assistant Rob to chauffeur him around. In fact, as far as he’s concerned, the main change is more spiritual than physical: “I have nothing else to fear,” he says. Pratchett has done a remarkable job in highlighting the appalling lack of research into Alzheimer’s (just three per cent of the funding granted to cancer), a disease which, terrifyingly, lies in wait for an estimated one in three of us.

Since he holds out little hope of any meaningful treatment being discovered in the time he has left, the topic that dominates our interview is assisted dying (although he says it’s not something that preoccupies him on a day-to-day level). His BBC documentary on assisted dying, Choosing To Die, caused something of a furore when it aired earlier this year for showing the dying moments of a motor neuron disease sufferer at Dignitas. Pratchett points out that almost all of the show’s 7,000 complaints landed before it even aired. So he has his own Dignitas forms on standby. “That’s more to show willing because it’s the only game in town but I’d much rather something would happen over here. When I was a boy people were hanged. I hope it’s not long before people say, ‘You mean, in those days, if someone was seriously ill, they just stuffed them somewhere with erratic care and, if they were good, fed them morphine?’”

Pratchett is very clear that there need to be stringent guidelines, that assisted suicide should only be available to those who are “compos mentis with a debilitating, incurable disease and want to die peacefully. To say that anyone can have it if they want it, I don’t think I can go there.” He is not a religious man yet for him assisted dying is “a spiritual thing, it’s the ownership of your own life” so his anger is directly squarely at the government for denying that right. “We live in a democracy yet we are being bossed around by the government and we should be bossing the government around!” he says, irritably. “I’d like to see a referendum.”

Nonetheless, he has some optimism about the prospect of change. “Once it was said that even discussing it with a relative could leave you open to prosecution. Well, I discuss it with my wife all the time.” As a man who has been knighted for services to literature, does he ever feel that his achievements risk being eclipsed by his role as an Alzheimer’s campaigner? “Me standing up and saying I had Alzheimer’s was the best thing I’ve ever done on this planet,” he says firmly. “Except sire Rhianna [his daughter] because no one talked about it. I didn’t expect all the hoo-ha but I was very gratified by it.”