How to fill the gaps in A Level maths knowledge(preparing for STEP)?

There are a few ways really.

One way is to go through your textbook(s) and read the chapters that precede the exercises. Then you can see how results are derived, which isn't usually shown in class and which is useful in STEP-style proofs. Another way is to go through as many exercises as you can in your textbooks, and then when you become stuck you can find out where you're stuck and unstick yourself using the book. Yet another way is to go through loads of STEP past papers and, instead of necessarily doing each question, at least look at it and try to identify the methods needed to do it. If you're unfamiliar with any of these methods, look through your textbook, poke around the internet, ask on here, etc... until you find a way to do it. Then do it Thinking about it, the last method I mentioned is probably better than the first two.

There are also other ways to patch up your knowledge... the possibilities are endless!

what a site, rep to you.