Nick - If you've taken the tests before then you can either retake them or choose to have your old results carried over, I think. There is a time limit though so you'd need to speak to OASC.
Harri - good luck. Most of the above advice is fairly sound, don't worry too much about the aptitude tests, they test for just that - aptitude - which you've either got or you ain't. Doesn't hurt to brush up on mental aritmetic (speed, distance, time, Peter leaves A at 0800 which is twenty miles from B and Derek leaves C at 0830 which is.....that kind of thing), also IQ tests get you in the puzzle solving frame of mind.