Follow the link in my sig to the Anthony Nolan Trust, there's details there. Or you can speak to the nurses when you go to give blood and sign up there. All they will do is test a sample of your blood and put you on the register, so it's painless if you're already giving blood anyway.
Jake - that's quite common due to the way they test the samples. The first tests give them an outline on the matches, but it's quite broad. If a search then finds you to be within that broad match they'll contact you for another blood test and do a much deeper DNA test that will fully show the match and whether you're close enough or not. They're always aiming for 100% match as this causes the least problems, though it is never devoid of problems.
Interestingly, most people don't realise that the matching process is done on something called HLA (Human Leukocyte Antigens) and not on blood group. Basically it's matching the immune systems to each other. The donor can be a completely different blood group to the recipient. I used to be O-Positive but my donor was B-Negative. Over 2-3 months following my transplant my body switched and I am now B-Negative.