Well I got an E in Maths at AS, turned it to a C at A2 (few marks off a B). Only reason I didn't get it remarked was because I got into Uni and I was so happy so I didn't really care.
Truth is, I did past papers from the first week I got back to college. Don't begin your revision for retakes next summer when you should be focusing on A2's, do them regularly from the moment you get back. I managed to get B for C1, A for C2, A for C3 and C for C4.
Don't be disheartened, just make sure you do past papers. Doing C3 really helped me understand C1 and C2 better (bizarre, I know, but thats the truth). Best of luck btw.