Well you should do past papers as well, not necessary but lessens the likelihood of messing up. My revision at GCSE was mainly textbook and a couple of past papers. Got 7A*s and for some exams e.g. my first history exam I barely looked at any past paper and just read notes and did well, got full UMS in that one if I recall. There are many avenues to great success in GCSE, it's GCSE after all. For Science used CGP almost exclusively, I did fewer past papers early on, with lesser UMS, then when I did past papers in preparation for later exams UMS was higher. So the lesson is..might as well use past papers. They will become bread and butter at A level