Hi Andrew,
When I was your age (11 years ago I was doing my GCSE's) I too wasn't sure of my level of mathematical understanding. Although I was in the higher group for GCSE maths I didn't exactly feel comfortable with the subject matter.
However I have found during the past seven years (since I started flying) that everyone has the ability to fly its just some people have to put in a lot more hard work than others (I'm one of them). The level of maths required isn't that high whilst flying. Ie whilst in flight as long as you can work out simple sums in your head then you should be fine. Ie you're not expected to know how to do trig from tables in flight (or elsewhere) in your head so relax a little!!!
In the ATPL Ground Exam Subjects (14 of them) you will need to know trig, algebra, equations etc etc... but not much, if any, higher than GCSE Mathematics, as you're looking to obtain a C grade then you should be just fine.
I would seriously suggest you ask your parents to take you to the Flight Training Exhibition at London Heathrow this coming Saturday.
At the exhibition you can talk to training providors, BALPA, CAA, take notes at the numerours forums that a run throughout the day.
http://www.flyer.co.uk/PFTSsoon.php
The best of luck with your studies now and in the future.
Best wishes,
Charlie Zulu