In the programme, Rob Wilkins,  who was collaborating on the unfinished autobiography, recalled the day in autumn 2007 when they both realised that ...“something strange had happened”.

He said that Pratchett had come into his office saying, “The S on my keyboard has gone . . . Come on, what have you done with it?”

Wilkins reveals that towards the end of his life Pratchett became increasingly angered by his disease. “He could see how it was affecting him, how it was tripping him up and I knew we were up against it for time. We had to get the words down and with that white heat, with that white anger driving him to write seven whole novels through the haze of Alzheimer’s.”

It was late in 2014 that Pratchett realised that his writing days were over.

“We had had a good day working on the biography and he said to me, ‘Rob, Terry Pratchett is dead.’
Completely out of the blue.
I said, “Terry look at the words you have written today. It is fantastic." And he said, ‘No, no, Terry Pratchett is dead.’”

The author, whose series of Discworld novels sold more than 70 million copies and made him one of Britain’s most successful authors, died in March 2015, aged 66.

In the BBC2 programme, Terry Pratchett: Back in Black, which stars Paul Kaye in the title role, Pratchett also sheds light on his “humble childhood” and why he became a writer.

The son of a mechanic and a secretary he was inspired by The Wind in the Willows and got a job in his local library which he said was when he “was probably at my happiest”.

He was bullied at school and had a “mouthful of speech impediments that left me with a voice that sounds like David Bellamy with his hand caught inside an electric fire. But it was not the kids that really got to me, it was the crushing of my boyhood dreams by someone 3ft taller”.

Pratchett says that Bill Tame, the headmaster of Holtspur School in Beaconsfield, Buckinghamshire, took a “rather vicious dislike to me” and thought that “he could tell how successful you were going to be in later life by how well you could read or write at the age of six”.

Pratchett was more interested in “climbing the desks than working at them”.

His novels, set in a world held up by four elephants balanced on the back of a turtle, were dismissed by critics despite their popularity with readers. In the programme Pratchett says that the “feeling of somehow being inferior” created from his school experiences and literary criticisms was “hard to shake off”.

This anger however, had carried him “quite a long way”. he explained. Of his knighthood in the 2009 new year honours, he added:
“Not bad for a boy who was told that he would never amount to anything.”

Pratchett made no secret of his diagnosis and campaigned tirelessly for The Alzheimer's Society.

On his Alzheimer’s, Pratchett wrote:

“On the first day of my journalistic career I saw my first corpse – some unfortunate chap fell down a hole in a farm and drowned in pig shit. All I can say is that, compared with his horrific demise, Alzheimer’s is a walk in the park. Except with Alzheimer’s my park keeps changing. The trees get up and walk over there, the benches go missing and the paths seem to be unwinding into particularly vindictive serpents."

“Imagine you are in a very, very, slow-motion car crash, nothing much seems to be happening at all, there might be the odd banging noise possibly, a little crunching sound here and there, a screw might pop out and spin across the dashboard as if it was in Apollo 13. But the radio is blasting rock’n’roll, the heaters are on and it doesn’t seem all that bad, except for the certain knowledge that at some point your head is going to go smashing through that windscreen.”

“I always dreamt that when I died I would be sat in a deckchair with a glass of brandy listening to Thomas Tallis on the iPod. But I had Alzheimer’s, so I forgot all about that.”