Piper,
I just sat the September exams and I'm pretty sure I screwed up General Nav because I didn't look through the paper before starting, to gauge the layout of the exam.
As HWD said, a computer selects the questions, although the CAA can tweak it to give a certain breakdown of question types which I believe they have recently admitted to. Guessing how that breakdown will go is like guessing the lottery numbers. If anything at all, make sure you are really quick on the Whizz Wheel. I cannot emphasise that enough. This month the Gen Nav exam was mainly CRP5 stuff and time was of the essence. Sadly I was not in the right frame of mind on the day and didn't quite finish.
If you know your chart differences/CRP5 (not just the straight forward stuff!) and scale calcs, PSRs etc you should be ok. We did have a bit of INS but straight forward G.Circle bearings at waypoints etc. I must admit I've tried to forget about it so my memory is a bit hazy! Also have a look at the gift questions like the highest latitude you can observe a sunset and sunrise, a straight line on a Mercator is...etc to give you extra time for the long whinded questions.
Instruments was pretty straight forward as far as I remember. Again my brain has experienced meltdown during last week so I can't remember too much specifically except, not much FMS or INS came up, a fair bit on Gyros was in there I seem to recall.
PM me if you need any more info.