I'll never understand why people have to be so absolute when they talk about this. Everyone has basically either said "It's the easiest thing in the world ever" or "it's impossible and you're going to fail".
The A-level is harder than the GCSE. It builds on knowledge you should have from the GCSE, but it gets harder. Although the core units increase in difficulty as you progress through the year, there is still a pretty big jump up in difficulty (and more noticeably, workload) between GCSE and the start of A-level. Like others have said, how difficult you will find it will depend on which areas of GCSE you were stronger or weaker at. But to generalise, I don't see any reason why someone working at your level should be unable to get a maths A-level. The important thing is to be ready at the beginning of the year, and prepared for the increased workload so it doesn't take you by surprise. Good luck.