This is one of my proudest creations as a Maths teacher and here I am setting it free. It's a spreadsheet I've put together from lots of documents around showing what skills came under old National Curriculum levels and GCSE letter grades. I've used those and my own judgment to give an overall idea where these skills fit into the new(ish) grading system. There are a few skills missing from this, especially towards the top end of the grades, and obviously many of the skills can be framed in a question to make them easier or harder than the grade suggested, so there's some professional judgement needed in using this. In addition, in my old school we graded students in KS3 based on their "trajectory" - so, we graded them according to whether they were on course to eventually get a grade 6, for example. The mythical student making "average" progress all the way through school and ending up with a grade 6 in Year 11 should be getting a grade '7.6...
Maths Resources, Explanations and Puzzles, for KS3, KS4 (GCSE) and UKMT Challenges