The difficulty of further maths depends on your aptitude for maths, how much you enjoy maths and the modules your school picks. If you have a low aptitude for math and physics and you don't enjoy math and your school makes you do all five mechanics modules (M1 + M2 + M3 + M4 + M5), it will be very difficult. However, if you enjoy maths and have a good aptitude for it, and if your school picks reasonable modules it shouldn't be too difficult. I only achieved an A in GCSE Maths and I'm currently in the first year of further maths and I'm coping with it pretty well - I'm achieving a mix of grades A and Bs in mock exams and class tests