org.uk is a vast resource of stimulating, thought-provoking activities, problems, historical titbits, games for the whole ability range. Large parts now linked to the national curriculum.
Book Reaching the Core of AS Mathematics (pound;50 including CD, pound;37.50 for members, from ATM. See www.atm.org.uk). Individual and group resources for developing algebraic skills in the context of solving problems, and 3 "big picture" programs. (KS5)
Website Bring On The Maths from www.kangaroomaths.com (pound;300 for 10 users).
This is interactive and imaginative material covering C1-4, part of a vast range of resources, some of which are free.