The system employed at Notts runs as follows:
Every grade you get accords you a certain number of points. For example, a first class mark in a 15 credit module gives you X points, so in a 30 credit module it gives you 2X points. Starred first marks (75+), which are HARDLY EVER awarded, get you extra points... but you cant actually graduate with a starred first I don't think, no-one has ever come close anyway if you can.
Then you need to add the points up over your second and third year to see where you are... if works out as follows. Pretend that grades are broken down into 16 15 credit modules (last year i did 4x30 creds, accounting for half of this), you need 7/16 firsts and 9/16 2:1s to get a first. This may sound easy, but trust me it just isnt'... and if you have 1 2:2 ur basically buggered. All other grades you need half of the higher class and half of the lower (ie 8 2:1s and 8 2:2s = 2:1). Theoretically therefore a 55% average can be a 2:1, again this all sounds very easy but when you bear in mind that last year there were 4 firsts out of 220, and 55% got 2:1s, it isn't as easy as it sounds.
On an interesting note, my points score for last year is exactly what you need over 2 years to get a 2:2, so that;s in the bag lol. When I opened my transcript and saw numbers like "28" I was like "no way, that's so wrong", and was half way to complaining when I realised I was looking at the wrong column and that that is a first class points score for 15 creds. It's relatively wounding thought when you glance down that column and see number like that though!