The Mastering Maths approach to teaching GCSE resit maths in FE is:
exemplified by twelve research lessons (on this page the is information about the design of the lessons and a brief introduction to each of the 12 lessons)
You can work through the research lessons using the self-study materials (Desmos).
You can read more about the impact on students here.
Sarah, who took part in the CfEM trial and the Mastering Maths trial, explains why she thinks the mastery approach works.