GS,
I finished my ATPL's just over a year ago, and had similar thoughts to yourself prior to starting the course.
I found it extremely beneficial to come up to speed with what you learnt on your PPL course. Go over each an individual subject and make sure you are comfortable with the subject matter. For me it formed some solid building blocks to tackle the ATPL course. Remember that the ATPL subjects are not rocket science, however, the sheer volume of material may scare you, but don't let it, you will get through it as long as you put the work in.
With respect to the mathematics and physics involved, once again, its not really beyond GCSE level. Bring yourself up to speed on things like, pythagorous, triginometry, speed distance time calculations, newtons laws of motion, and basic thermodynamics. For you own benefit too, get hold of a CRP-5, and start using it to solve NAV problems, you need to know how to use this dam thing inside out for the General Nav exam.
Good luck, and enjoy it!