I agree with maxdrypower.
Start them now, and concentrate on the Airlaw. That way when you come to Solo you wont have to worry about that exam, it will also help with the early stages of flying.. See exams like the Nav exam really are best done towards the end of the course when you have a practical understanding and can use that in the exam.
Also get the Oxford cd's I bought one which was a preparation CD of the Skills test.. It's brilliant to start off with also as it explains things like " power attitude trim" straight and level, coordinated turns and it's a real bonus to go into those early lessons with a good understanding about whats happening in the plane and it's happening.