Russell,
the point of aptitude tests is to find out whether you have the basic motor abilities that are required in flying. If you do, you will pass. If you don't, you will fail. Simple. Yes, there are borderline cases and, in general, they will not be invited to continue - would you want to be flown by a pilot with borderline aptitude?
I appreciate that you'd like to get as much knowledge as you can before you present yourself for assessment, but this particular aspect you are unlikely to be able to do amything about. So don't worry about it!
I'm sure, having said that, that many here will give you lots of pointers to the RAF's crrent recruitment process; for myself, as one who used to be actively involved in that process, I'd say get yourself as knowledgable as you can about the RAF, current affairs (and have an opinion on what's going on...), the latest defence programmes and their states of play (JSF and Nimrod 2000 are two examples), that sort of thing. Decide how you would explain what contribution you could make to the Service - why should they employ you as opposed to someone else? Have a feel for what the moral arguments facing a military service are - should we be in Kosovo, for instance, what are the arguments for and against?
There's a lot to consider, and no-one will be expecting you to be able to take CAS's job the day after tomorrow, but you need to be positive, aware, and enthusiastic. Let the aptitude bit take care of itself!