For some excellent PowerPoint resources with A Level questions and mark schemes by topic try Mr Westwater’s resources on TES. (Twitter @westiesworkshop). Though Edexcel questions these are clearly useful for other examination boards also. There are six of these PowerPoint files available:
For any of your students studying for STEP direct them to this excellent portalfrom stepmaths.co.uk which has (all free) access to STEP questions and solutions. Create an account, login and you have access to a complete library of resources. For further details see Mathematical Miscellany #10
For an extensive collection of Challenging questions for Advanced Level students we can turn to the Oxford Maths Admissions Test, note the menu on the right hand side of the page, many papers and also solutions are available. For more Oxford MAT questions see this extensive collection on Underground Mathematics. These Underground Mathematics resources will provide so much more than the question in each case. Note the suggestions and solutions.