DD has asked me to see if anyone on MN can assist (maybe @noblegiraffe)? The teachers give them lists of what to buy, for regular maths easy as there's standard WJEC approved textbooks. Apparantly for WJEC further maths there's no official textbook, because there's not enough students to make it worth while publishing.
For FM mechanics the teacher has recommended a couple of books that between them cover all the topics needed. However the teacher for FM pure hasn't suggested anything (although a few of them have asked). DD really likes having a textbook to refer to and also to have extra examples to work through, and would welcome any suggestions, even if it doesn't cover all the topics something would be better than nothing!