@user8800 we are finding the same thing at our school. There are no timetabled end of year exams but some teachers are doing them, with little notice and no time to revise due to homework, other end of topic tests etc. Some subjects, no idea at all.
Thanks for all your suggestions, really useful. I've gone on the website for physicsandmathstutor.com and went through some flashcards with DS who found it "boring" but I will print out some practise questions for him to do at the weekend. He has done Seneca for Geography as instructed by the teacher, so we did do that for Biology too and he seemed to get on well with it, although it did take a long time to complete one topic! But I think rather than cram everything in it's better to carry on with the Seneca if he prefers that even if we only cover a couple of topics.
Also, Dr. Frost has been great for us. Both DCs like the question format and it's good that you can choose questions according to what they need to work on and the right level of hardness. Both of them have done really well in maths so far using only that for revision.
Another whinge at the school....DS is doing PE GCSE and the sports dept is RUBBISH. I usually have a lot of sympathy for teachers as DH is a teacher and I know how much they are under pressure. But, no guidance on anything, won't answer to emails most of the time, no notice for exams, no revision material or helpful aids suggested, no clue as to how to film their sports or how good they have to be to get a certain grade. It's ridiculous. We've been googling for information about how to produce a cycling video for assessment, no help from the teacher at all, despite several emails! Cycling and also cricket are summer sports so have to get footage now, not in the autumn! DD who is going into year 10 in the same school has also chosen PE as she is also very sporty, but I'm regretting it now!