I literally got 30% in the FMSQ exam two years ago (which came out as a D), it's not that big a deal - it won't affect your chances of getting into sixth form/college/apprenticeships, and you don't have to delare it on your UCAS form if you apply to uni. I wouldn't worry about it, especially if you're not taking A level maths. On the bright side, it did make the GCSE maths exams seem easy!