Honestly, it doesn't really matter. I took Chemistry and Maths (along with further maths and physics), and Chemistry (AS at least, I can only speak from experience) definitely doesn't required the majority of the maths that you will learn at a-level. For my exam board (WJEC), the first topic was fairly maths based, however it's nothing you either should know already from gcse maths or that you can easily pick up and learn just like anything in chemistry! So I would say, take maths a-level if you want to, but if you don't enjoy maths or find it difficult to understand then don't bother, you don't NEED if for chemistry, 100%.