GNAV, MET and FL PLAN I do think that you need a good understanding of the core subject, attending the full time residential ground school directed me in the right direction for the others. It was amazing when doing some of the questions I could remeber the lecturers teaching that subject or even a word or phrase would come to mind.
I think the JAA exams are being refined at the moment, all schools and JAA states have to submit propositions for a consultation process to stop duplication over subjects eg compass swinging in gnav and insts, also a lot of commonality between systems and POF etc.