Capt.Biggles,
Are you talking about the Guild of Air Pilots and Air Navigators Tests (GAPAN) which are done at RAF Cranwell????
As far as preparation for the computer based testing goes there's not alot you can do really. The tests are pretty unique. Get hold of a copy of Junes Flyer, there was a good article about it in there which I would recommend you have a look at. Your local club will have back copies knocking about no doubt. As far as a pass mark goes you either pass or fail. They don't tell you the score you got even though they do score you so should you sit the tests again you can be compared to your first attempt.
At the end there is an interview which I found to be quite relaxed. They asked me about my school days, uni, etc..... what aircraft I flew which they then quized me on the V speeds.
Hope this helps and good luck if you have a crack.
Autofly