Hi Greg737
Your situation reminded me very much of myself a few years ago. I got my PPL in a Robin at 17 and as I see you're in Dorset i'm guessing you might be flying from Bournemouth too. I don't know if they're still going but they used to run some courses at BCFT in the evenings which are very good if you can get there, and, like others have said the PPL confuser is very good! I know it is 'another book' but it has some very useful past papers in there which contain many of the same questions as are in the actual exams.
I had the Oxford CDs and found them useful for the navigation and some parts but would suggest that ground school is more valuable. Most instructors I've come across are fairly happy to help you out with ground school, especially the odd evening or when the weather isn't good enough to go flying.
Good luck with your exams, it's certainly not easy balancing flying, A levels and everything else!