Take it! If you know you are hard working, take it! The reason why people who got stellar grades at gcse then go on to college to get d/e at a level chemistry is they didn't revise enough. At GCSE you can revise 2 weeks prior an exam but at A level if you do this you're gonna get the shock of your life. It's simply understanding the concept, lots of past papers, and regurgitating information under exam conditions. Admittedly I have found A level biology harder than chemistry due to the ruthless marking of Aqa. Accuracy is essential.