I sat the Gen Nav exam in Dec 2003. Much was written about that paper because more people failed it then usual apparently. They failed because their CRP 5 work was not up to speed. 26 of the 54 questions were on CRP 5 and none of the questions in the paper on CRP 5 were above what you are taught for your PPL.
Know how to use your CRP 5 speedily and accurately and you will go a long way to passing this exam. If you can grasp the stuff that Send Clowns has written on this thread on the rest of the Gen Nav stuff you will be fine. The key to the form of the earth type questions is to draw a picture. The INS one is a classic - if you draw the picture you will be able to work out that only 1 of the given answers is possibly correct.
Best of luck, I got 90%.