This is a film I'd heard of years ago, but never got around to watching it until yesterday.
While the plot seems a little slow at times (an illusion created by watching too many hollywood action movies), and (as many people here have commented) the dialog can get a little cheesy, add to the fact that the hairstyles, clothing, vehicles, etc... look dated now. But that's not what I was watching the film for.
The actual action sequences in the film are typical special operations style action sequences, i.e. they're over practically before they've started! This film, if not realistic (I'm sure it is, but as someone who doesn't know how the SAS really operate, I couldn't comment), is totally believable. From the handover by the police to the military, through the planning phase of the operation, the MOE (fast-roping from helicopters onto the roof, then abseiling down the side of the building), the total communication throughout the assault. It's absolutely breathtaking.
This is no Hollywood Action Movie. It's totally different from the likes of "Operation Delta Force", etc... There are no cheesy one-liners when a terrorist is dropped. There is no jumping about, emptying whole magazines (and more!) into people, shooting from the hip at targets 200 metres away and hitting them with the first shot, while snipers on the opposite side couldn't hit the Titanic from 2 metres away.
So, to sum up, I'd give the plot, characters and acting 7/10, but the action sequences, I'd wholeheartdly give 12/10! Fantastic stuff...