The OCR GCSE (9-1) Maths has many resources including the specification and practice papers.
A very useful feature of the OCR Specification is the ‘initial learning’ column showing the basics which should established with learners before moving on to more difficult areas. This is very useful when thinking about our teaching for younger students.
On Diagnostic Questions OCR GCSE Multiple Choice covering the specification are available – an outstanding resource.
From OCR, it strikes me that the Check in tests will be very useful in KS3, consider the language of functions for example, the first questions could be used with students as young as Year 7. Full details of the Check in tests can be found in the Teachers’ Guide. Each test is of a similar format in that Questions 1-5 cover procedural calculations (AO1), questions 6-8 require the ability to reason and communicate mathematically (AO2) and questions 9-10 relate to problem solving tasks (AO3). There is also an extension task. Very usefully (thank you OCR) the Check in tests are also available in Word Format.
Staying with OCR, note the Delivery Guides which include useful links to resources and a series of PowerPoints including questions with answers (always so useful for busy teachers!).
I used the questions from this presentation with Year 7 (age 11-12) when we were studying highest common factors and lowest common multiples.