GCSE Maths confusion. What do you think is best?

Hey guys,
I was given the offer of re-sitting GCSE maths and thought why not since I was give the chance to change my C grade to hopefully, fingers crossed an A/A* (free of charge). But for some reason I am finding things difficult in terms of knowing what to study for GCSE maths to consolidate my knowledge- shifting from C to A/A*. In AS maths there were official books but with GCSE I tend to waffle off- doing questions that aren't in the spec though it is still algebra, percentages etc.
It seems that I don't know what to go over in order to consolidate my knowledge. Maybe this is where the specification comes in. This is going to sound odd but should I only focus on past paper questions? I began making my own questions which are harder than you'd normal get in a GCSE exam since I'm currently doing AS maths. I was told is was time wasting though because it's stopping me from moving on to finish going over/ learning all content since it means me taking more time to finish the questions I've made.
Due to AS maths, when it comes to Algebra etc I'm fine but stats it's a different story as I studied mechanics and avoided learning stats in year 11.

So my question is, should I only focus on answering questions in the past papers format or should I still go beyond what is needed since it prepares you for anything in the exam? Should I focus on the specification also or is it too vague meaning I should go beyond that? Many thanks!

Complete past papers, use MathsWatch and/or MyMaths. You could concentrate on A*-B topics only - the grade C stuff should click in as it's common sense.I am actually confused; why do people not pass GCSE Mathematics first time round? It's not that difficult.Wait... you do A-level Maths with a grade C at GCSE?

MathsWatch - where can I find info on this? Is it an App or is it on Youtube? I think it would be good for my son.I googled it and and came up with an App but it was asking for school login details so, I guess this is something different?

I bought mine from someone on eBay for £4.99 (as a download) which is considerably cheap. I'll send a link via PM.