I did the RAF aptitude tests once when I was 16 and failed and then again last year and passed. The first time, I did the whole lot - for all four branches and was offered a 6th form schol in ATC - not quite what I had in mind!
The digit recall one can be quite hard but I got the best result on that. Just remember the digits in smaller chunks - say them over quickly in your head in a tune - like for 7 digits, remember the first 3 and then the last four. Makes it much, much easier than being confused by a big bunch of numbers!
I remember being pretty baffled the first time - but if you have a clear head when you go in, it helps. They're not particularly hard and I didn't do much preperation - but I did practice quite a lot for the numerical questions which you can work out on paper. That helped. But hey, best of luck. What is this GAPAN thing you're going for?