Sir Terry Pratchett cannot help wondering why it was this year - after 30 years as a bestselling writer - that he was honoured with a knighthood: 'All I know is that on the citation it says 'for services to literature', and it would be nice to think that that got me the knighthood - though it may have been for what I stood up for and what I've done for Alzheimer's.' All in all, Pratchett is treating his recent knighthood with modesty: 'I am 60. I know exactly who I am,' he says. 'I am just me - which is why it's slightly amusing to be addressed by the postman as 'Sir Terry'.

In the year since his diagnosis of early-onset Alzheimer's, Pratchett, the nation’s best-loved fantasy writer, has campaigned tirelessly to increase awareness of the disease and government funding for research into it. He has personally donated over half a million pounds to Alzheimer's research.

He's also made an implacable effort to fight 'the demon', as he calls the disease. But on the day of our interview, he's in a more contemplative mood. 'Look at this contraption,' he says, at the house in Wiltshire he shares with his wife, Lyn. He points to a menacing-looking black object, the size of a human head, which was designed for him by an American Alzheimer's specialist; the wires apparently shoot electric pulses into the brain to encourage the growth of nerve pathways. Pratchett spent 2008 experimenting with different therapies; one of the first actions he took after his diagnosis was to have his mercury fillings removed and replaced with less toxic porcelain.

Pratchett suffers from a rare variant of Alzheimer's known as posterior cortical atrophy (PCA), which appears to have spared his fertile mind from the disease's usual crashing wave. So when he looks around and asks for a second time, 'Did I drink my tea?', he puts the memory lapse down to the rushed way in which the tea was drunk. 'See, Alzheimer's can make you distrust yourself when in fact you may be right. Sometimes it is just old age. And no one is more neurotic about themselves than an author.

He's aware critics are scrutinising his latest novel, Nation, for traces of the illness, and coming up with metaphorical interpretations of his battle: 'I don't know what they're expecting to find, but they won't find it.' Yet there are things he cannot put down to age, and he admits to having daily struggles with the illness: 'PCA is like being shackled to a joker; you don't know whether you're going to be incredibly sharp or miss something altogether.'

His past year has been unsettled by this sort of worry: 'But we had a better Christmas this year than last, when the Alzheimer's was hovering in the house. We hadn't got things squared in our minds. And I didn't have a specialist then, which was the cause of a lot of my anger.' Though he has felt very alone, the illness has, he says, 'made my wife and I closer. Woe betide me if I struggle with the lid of a jar; Lyn will rush to my aid. She's been so good'.

When he describes living with Alzheimer's as 'a minor flaw in a good, though complicated, year - without it, it would have been a fairly anodyne one', you can't help feeling he is underselling his recent literary achievements. Nation, his new book, was an instant bestseller; his Discworld series celebrated its 25th anniversary; and he was at work on two new titles: Unseen Academicals and I Shall Wear Midnight.

Not usually a man who's lost for words, the only one he could muster on the day his knighthood was announced was 'Flabbergasted!' Today he says: 'That still applies! But seriously, as a science-fiction writer, it is an achievement. Despite its popularity, sci-fi is still a ghetto genre. So when a hand of welcome comes from the Establishment, you can do nothing but shake it.

Interview by Josie Ensor.

Main portrait: Laura Pannack

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