When Terry Pratchett passed away in 2015, his friend and collaborator Neil Gaiman assumed their dream of a screen adaptation of Good Omens, the novel they wrote together in the late '80s, had died too.

Then he was contacted by Pratchett's estate. The late Discworld writer had left Gaiman a letter. In it, Pratchett asked his fellow author to set aside their previous agreement that they would both have an input into any television or movie adaptation of Good Omens. Gaiman should go it alone.

Four years later, the quest to bring Good Omens to TV is finally about to reach fruition, with Amazon Prime's six-part retelling of this apocalyptic comedy. It stars Michael Sheen and David Tennant as an angel and devil who, after centuries on earth, have struck up a comfortable friendship.

Alas, their cosy existence is about to the ripped asunder by the coming of the apocalypse. Either they follow the divine plan - or do their best to stop it. Cue slapstick, elements of horror and some pointed commentary on the absurdity of religion.

If the premise sounds relatively straightforward, the journey from page to screen was anything but, Gaiman has reported. For years he and Pratchett banged at the doors of producers - only to be variously misunderstood or straight-up rebuffed.

"Pratchett and I could both picture this thing," Gaiman would tell Entertainment Weekly. "We went out and talked to a lot of writers of television we admired and said, 'Would you like to do this?' They explained that they wouldn't, because they couldn't quite get a grip on it, and couldn't see how you could do it without changing it too much.

"I was probably ready to just let it slide but Terry wrote to me and said, 'You're the only person who has the same amount of love for Good Omens's I do and you understand it and you have to make this' Which left it as this awful last request. I had to do it."

The roots of the novel, which was published in 1990, reside in Gaiman's admiration, as a struggling young writer, of the older Pratchett. By the time the book was published, Gaiman was the hottest thing in the comic business, by dint of the Sandman comic series.

However, as with every overnight success, he'd endured a long period of struggle in the wilderness. During that time, he had eked a living in the most humiliating fashionable imaginable - yes reader, he was a journalist - and in this capacity had interviewed Pratchett.

"Terry Pratchett and I met in February 1985, in a Chinese restaurant," Gaiman would write. "I was a young journalist. He was a former journalist and Electricity Board PR, and a writer who had just published his second Discworld novel. I was the first journalist who had ever interviewed him. I remember we made each other laugh a lot. We laughed at the same things. We became friends. It was easy."

Gaiman was also obsessed at that time with Hitchhiker's Guide To The Galaxy author Douglas Adams and his ability to fuse the longstanding tradition of British humorous writing with the conventions of a genre novel. So he had a go himself and started writing a 'humorous' horror tale. He sent 5,000 words to Pratchett, who came back later with the offer either to buy the manuscript outright and use it as a springboard for his own novel, or to collaborate.

"Write it together," is how Gaiman recalled his response. "I was not stupid, and that was the nearest I was ever going to get to Michelangelo phoning to ask if I wanted to paint a ceiling with him."

Good Omens thus saw daylight at the dawn of the decade and quickly acquired a cult following. It succeeded because it inhabited a liminal spot between Pratchett's sardonic tweeness and Gaiman's darker, more jaundiced view of the world.

That same heightened spikiness characterises the TV adaptation (judging by the trailers). It also expands on the universe the authors created, with Mad Men's Jon Hamm playing the Archangel Gabriel who popped up in a sequel that Gaiman and Pratchett had planned but never completed. Gaiman's work, it should be acknowledged, has a patchy history when it comes to screen adaptations. Amazo'Õs take on his American Gods novel is visually sumptuous, frequently barking and occasionally incomprehensible. Far more watchable is the Tom Ellis-starring take on his Lucifer Morningstar character. And yet, Lucifer, which has just returned for a fourth season to Netflix, primarily succeeds because it's far, far cheesier than the source material. And of course a Sandman adaptation has been in the works for decades.

Can Good Omens triumph where previous Gaiman projects have come unstuck? (with the exception of the BBC's Neverwhere in the '90s of course). That obviously remains to be seen - though it is certainly faithful where it matters, as in the prominent positioning of Queen in the David Arnold-curated soundtrack.

"Terry used to joke that you can never remember buying Queen's Greatest Hits," Gaiman recalled. "But if you leave any cassette in your car long enough, it turns into Queen's Greatest Hits. I put Crowley listening to 'Bohemian Rhapsody' in his Bentley into the novel."

Good Omens is on Amazon Prime now.