If you got atleast a grade A at GCSE maths, then it's safe to say that you are able at a-level maths. Not to say though, that you will get a grade A for a-level maths just because you did at GCSE, no. Getting an A at a-level maths requires 10x the effort you put into GCSE maths. That includes reviewing what you learn each lesson and doing practice questions based on what you did in that lesson. You MUST be able to clock the lower grade questions (E,D,C grade questions) in a-level maths. Once you clock these, you just have to focus on the grade B, A and A* stuff. And that's when you do past exam papers and focus on those particular questions. On average, most grade A/A* candidates for a-level maths did atleast 3 hours of maths a day (of course, with breaks).