Fantasy's big at the moment. Everyone's looking for the new Harry Potter, which is why we've got adaptations of Narnia, Eragon, The Dark is Rising and so on being churned out by the studios. It would have been evil and wrong if Pratchett hadn't got a piece of the action and sure enough, on Christmas 2006 he did. Thanks, Sky One. What's more, they clearly threw plenty of money at it. There are a few dodgy special effects, but basically this is a sprawling extravagent love letter to Pratchett, full of famous British actors and faithful to a fault.

The problem is that it's too long. It's more than three hours, which Pratchett absolutely doesn't need. Fidelity is good, but here they're being faithful to an author who's never been particularly plot-driven in the first place. Episode One I loved, but Episode Two needs to lose a good thirty minutes. You'd hardly even need to make any cuts. You'd just have to tell everyone to get a move on. The stuff at the Tooth Fairy's castle got a bit boring, but even worse were the dramatic climaxes. I realise Pratchett is more interested in ideas than showdowns, but they're still there in the story. Here they're not even trying to make the most of them. There are three or four big confrontations towards the end and the incompetence of their realisation is almost shocking. Teatime's literal downfall is so badly edited that it hardly even makes sense. The threats aren't sold. There's almost no danger or excitement. The director's let himself get locked into a languid pace that's very Pratchettian and works for charming character scenes, but sucks donkey balls when it comes to the showdowns.

The only exception is Death, who's the coolest character in the story and becomes spine-chilling when confronting the Auditors. Boy, did those dudes pick the wrong guy to piss off. He's awesome, he gets all the best lines and he makes confrontations dramatic just by showing up for them. He's voiced by Ian Richardson, who even gets a throwaway gag referring to Francis Urquhart. "You might very well think that I'd already thought of that, but I couldn't possibly comment." Obviously I thought Richardson was great, although I'm also looking forward to Christopher Lee in the 1997 animated versions and oddly also Sky One's later The Colour of Magic. Or if we're being frivolous, perhaps they should have resurrected William Hartnell, since this Death is a time-traveller with an extradimensional home and a granddaughter called Susan.

Episode One suffers from the same longeurs, but there I quite liked it. It's the beginning. It's rather refreshing to see a modern piece of telefantasy take so much time pottering down all these alleyways. This isn't so much violating Robert McKee as kicking him to death with hobnailed boots. The story takes an amazing amount of time in getting to the point. We follow all these different characters as they go about their business and only when it's good and ready does the film let us begin to guess at who's going to be the hero. You remember all those people who criticised the first Harry Potter film for not being cinematic enough? Hogfather might actually kill them. I imagine this could be boring if you're not in the mood, but personally I loved peering into all the nooks and crannies of Pratchett's imagination. It's characterisation for the Discworld itself. Gradually the threads coalesce into a story and I found the process fascinating.

Episode Two desperately needs to lose a good third of its running time and no one would ever complain, but I think it would be a mistake to do the same to Episode One. It could be done, of course. Frankly, even a two-hour Hogfather might be a little on the long side. It's full of random nonsense like Ridcully's attempt to use Bloody Stupid Johnson's bathroom, but even so I think Episode One works. It's eccentric, funny and capturing Pratchett's essence in a way you wouldn't get with the efficient 90-minute Hollywood version.

What I like best about Pratchett is his way of thinking. Like Douglas Adams, he's an extremely intelligent man with a distinctive philosophy and an interest in exploring lines of thought. Pratchett's books will often be full of ideas and this film manages to communicate that. The magical principles of belief are a fairly well-known fantasy idea, but what's more unusual is the lengths to which Pratchett takes them. One of my favourite scenes in the film has Death asking a computer if it believes. How many films would even think of that? Here we have the pagan origins of Christmas and Death drawing a line from Santa Claus to the existence of justice and mercy. There's a ferocity to Pratchett's ideas and the film's much more successful at conveying those than at doing conventional adventure drama.

I should discuss the cast. David Jason is better than I'd expected as Albert. There's a reason every British TV show under the sun wants to cast him, you know. The wonderful Marc Warren sometimes seems almost trapped inside his characterisation of Teatime, but he hits all his notes and I'm still a big fan. They've made him look like an evil Frodo. Michelle Dockery has the toughest job as Susan, since the original character is as chilly as the North Pole and frankly not very likeable, but I suppose she did okay. I'd have liked her to show a bit more casualness with her grandfather, even if only in selective moments, but I suppose neither character is exactly cuddly. Their reunion's meant to be awkward. I loved her Bride of Frankenstein hairdo, though. (Instead of black with a white streak, she's white with a black streak.)

Then you've got David Warner, Tony Robinson, Nigel Planer... the only one I didn't like was Nicholas Tennant's Corporal Nobbs. It's not his fault, mind you. Nobby's meant to be a revolting disreputable creature that needs a certificate from the Patrician to be accepted as human, but this Nobby simply has big teeth. Er, no. He should have been older and scuzzy. Bizarrely they've left in a joke which requires the original Nobby and doesn't work with this version.

Oh, and the last scene stars Pratchett himself, as an actor. He's only in it for a minute, but for me it was the perfect ending.

It looks great, mostly. There's some poor blue-screen work, but otherwise this is a design triumph. The title sequence is a wonder, zooming in from space, past A-Tuin and all the way to Ankh-Morpork until it reaches a particular window. The Assassin's Guild looks gorgeous. Hex, the magical computer, is a treat for the eyes. Something like the Death of Rats is the kind of thing you'll get nowhere else, a random offshoot of Pratchett's fictional universe evolving as he plays with it. What's more, it's as mad as toast, but it also holds together and makes sense.

Episode Two is frustrating, but I even so I really appreciated this adaptation and its languidly insane faithfulness. They push the style so far that it breaks, but when it works I think it's something special. There's something extraordinary about a morphing picture on a Hogwatch card, for instance. The adaptation is deeply flawed on a dramatic level, especially when it comes to those confrontations, but the scene of Death vs. the Auditors almost manages to make up for it. Besides, despite the eccentric structure and pacing, this still basically works as a story. You've got a surprisingly brutal Episode One, with Teatime scattering corpses like confetti, and Death is a complete star. He's funny. "Onwards, Binky." He's cool. He's even oddly moving when he saves the match girl.

I can't wait to see what they'll do with The Colour of Magic. Those two early books don't have a plot at all.