I am reading over my ATPL books now (so I know what to expect) and I wouldn't say the maths part is as hard as 'A' level standard, but with a good grade in 'A' level Maths and Physics I don't think ATPL maths would be worth stressing over.
I have not yet finished my ATPL books or even sat an exam yet, so I could be totaly wrong in saying that the ATPL exams would probably use complex maths that would rate somewhere betwen a higher GCSE paper and an 'A' level paper?
Andy
ex - GCSE and 'A' level Maths student