The Good Omens finale takes a major detour from the novel – and that's a good thing

And yet – according to the progress bar creeping along the bottom of the screen – there are still 30 minutes of the episode left to go. So what now?

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We soon have our answer. What follows is completely uncharted territory, because showrunner Neil Gaiman has inserted a whole extra chunk of story into the final episode. And it's brilliant.

As the angel Aziraphale (Michael Sheen) and demon Crowley (David Tennant) meet for an ice-cream and a friendly catch-up in St James's Park, they are kidnapped by the agents of Heaven and Hell and bundled off Upstairs and Downstairs respectively.

Unsurprisingly, Head Office is not pleased with either of them. For consorting with Crowley and averting the End Times, Aziraphale is to be killed by fire; meanwhile, Crowley is subjected to a show trial with the judge from Hell (literally) and sentenced to be dunked in a bath of Holy Water.

But, to the shock of Angel Gabriel (Jon Hamm), Aziraphale is now fire-proof and sits grinning manically in the middle of the blaze. Likewise, Beelzebub (Anna Maxwell Martin) is horrified and humiliated when Crowley fails to dissolve into a screaming glob of nothingness – instead splashing around in the bath and requesting a rubber ducky. Something seriously weird has happened.

Before we know it, Heaven and Hell have washed their hands of the whole business. Crowley and Aziraphale have been dumped back on Earth, and now we find out the secret behind their survival: they swapped bodies.

Having pulled this off, the duo quickly switch back into the right bodies and head to the Ritz, where a table for lunch has mysteriously become available.

Of course, fans of the book will know that Aziraphale and Crowley reunite in St James's Park where they have a brief (but significant) chat about the "ineffable plan" and then go to lunch. But in the novel, all we actually hear about the Heavenly and Hellish responses is this:

Crowley gave him a sideways look. "Your people been in touch?" he said.
'No. Yours?'
'No.'
'I think they're pretending it didn't happen.'
'Mine too, I suppose. That's bureaucracy for you.'
'And I think mine are waiting to see what happens next,' said Aziraphale.

Neil Gaiman has said that he changed the ending of the series so fans of the book "wouldn't get cocky", blindsiding them with a new addition to the story – but there's a more prosaic reason, too: when dividing up the novel for episodes, he simply ran out of material.

"When I got to episode six, I realised that I ran out of story halfway through, so I was going to need more story to take me to the end," he told the audience at the Good Omens world premiere in London.

Whatever Gaiman's reasons for the change, episode six's detour into Heaven and Hell is a success – because it manages two important things simultaneously.

Firstly, it works because it gives us a greater insight into the relationship between Crowley and Aziraphale, which is at the heart of the whole story.

Gaiman's other major addition (which wasn't in the book) is the episode three "mini-movie" which sprints through the entire history of the world and charts the progress of Crowley and Aziraphale's relationship – and this extra section in the series finale gives us a satisfying ending to that story.

The body-swap storyline also shows us just how close Tennant and Sheen's characters have become, to the point where they can perfectly mimic each other. Like the series as a whole, it invites us to question the dichotomy between good and evil.

Secondly, Gaiman has managed to pull this off without massively disrupting the plot (which is good, partly because the plot is already great and partly because fans would be FURIOUS if he did).

As Good Omens comes to an end, Crowley and Aziraphale still end up back on Earth with no real repercussions from their respective bosses – and it's time for lunch at the Ritz.