Sadly, despite the rich source material, the rest of the film is surprisingly unfunny. With three series of radio programmes, providing something like nine hours of script, alongside the subtly different three hours of TV shows and the contents of five books, you would have thought ‘The Hitchhiker’s Guide to the Galaxy’ film would be filled to brimming with comedy broth, and that the whole thing would fly past in an agony of aching ribs and splitting sides. Sadly, whoever took the chainsaw to the final version (one Karey Kirkpatrick has co-writer credits with Adams) obviously decided that Douglas Adams wasn’t much of a writer and that they’d better add some new scenes for padding and change the end, ditching most of the pointless funny bits along the way. True, they make people squirt milk from their noses, even if they’re not drinking milk, but do they advance the plot?

So, for the benefit of the dumb cinemagoer, who it is assumed has the intelligence of a rubber stamp, the story is tightened, Arthur’s love affair with Trillian is turned up a notch, the Vogons are changed from comedy bureaucrats and cruel caricatures of transport planners to The Bad Guys, a new female character is added to address the obvious gender imbalance that so ruined the original, John Malkovich plays a religious leader to no great purpose or end, generating no laughs, and Zaphod has his second head removed halfway through. Bastards.

Still, bits are good, which is to say the bits that they haven’t rewritten or inserted, the bits that Adams actually intended to be in there. And the cast, by and large, are enjoyable – Martin Freeman was a natural choice to play Arthur Dent, Tim from ‘The Office’ being a kindred everyman, and he does it well, despite not seeming embarrassed enough when forced by the script to jump artificially into hero mode. Stephen Fry’s is exactly the voice you’d like the Guide to have were it ever to actually exist, and similarly Alan Rickman is Marvin – the ‘Paranoid Robot’ apparently. A friend suspects Thom Yorke now owns the copyright and they were unable to use ‘Paranoid Android’ in the film.

Sam Rockwell is surprisingly good as Zaphod, obviously having a lot of fun in the role, and playing with a truly impressive lunacy, even semi-decapitated as he is. In fact, he’s a match for Mark Wing-Davey, the original Zaphod; a real force of madness. But Mos Def can’t act, and his Ford Prefect is reminiscent of a badly staged high school play on a tight budget and with no time for rehearsals – his is simply one of the worst performances I can recall seeing in a real film, and his presence is baffling. It seems as if someone had the bright idea of making Ford black, fine in and of itself, but then figured that one black actor’s pretty much as good as another. Or perhaps they forgot to audition him – easily done, I suppose. These things happen. But he kills Ford, a lynchpin character and fan favourite, and renders him a supporting figure. If I saw Mos Def in the street I suppose I’d throw small rocks at him. Trillian meanwhile is made much more capable and not at all a ditzy blonde, perhaps to make her ‘love story’ with Arthur more pertinent, but leaving you wondering why on earth she ran off with an idiot like Zaphod in the first place.

It’s a patchwork movie – bits are good, others are awful, and the whole thing is a mess of frayed ends. This actually makes the good parts almost unbearable, since you can see what a great film this could have been – and should have been. Once again I wonder how anyone can take a by and large excellent cast and one of the greatest of source materials and manage to make such a botch of it. Yet it seems to happen repeatedly these days. Sad.