I've taught people to get through Cambridge language exams, and one thing I noticed - especially at the lower end of the scale - was that the candidates' level of English was only equal to their level of competence in answering the questions the way Cambridge required in securing a good grade.
Key skills like concentrating initial on eliminating wrong answers before looking for the right answer are massively influential on how well a child will do. I took the 11+ in the early 70s and whilst I think I had some coaching at school beforehand, it can't have been much.
Anyway, thanks for the replies. I've ordered those books and will look at the website with my daughter.