MDR
A C in GCSE maths is a minimum. Having a GCSE in maths isn't a guarantee that you'll be good enough either.
Today's school taught mathematics is all calculator and the like and very little mental arithmetic. MDR being the mainstay of the majority of aircrew. As far as I'm aware, all services to teach mathematics as a part of the aircrew course (RAF defintely do) but its all geared directly at teh type of mathematics you'll be using such as speed, distance time and interpolation etc.
A good book would be Mental Maths for Pilots.