I got full marks and close to full marks for core 3 and core 4.
I was OCR.
I would actually do all the questions in the book and actually understand the content if you want a*. To get A overall you just need to do know how to do most of the questions, and they don't change much from year to year so its pretty safe.
To get full marks or close, just revising papers won't do it for you because the last 2 or so questions are specifically designed to be different.
So you need to understand what your doing. If you're differentiating a trig function, most people just revise the derivative of sin x is cos x, but if you actually take time to study the graphs of these various functions and think about why the result is so. Same for things like the exponential function, its amazing but most people just look at it as e to the x and nothing more.
Then when you get to the questions at the end, you will visualize exactly what they're trying to ask you.
Also you are very lucky, in essence the questions and the format is so easy at A level compared to maths at uni. There's no 'deep' thinking, its straight forward.