In 2007, the Nuffield Foundation commissioned a team from the University of Oxford to review the available research literature on how children learn mathematics. The resulting review is presented in a series of eight papers.

Papers 2 to 5 focus mainly on mathematics relevant to primary schools (pupils to age 11 years), while papers 6 and 7 consider aspects of mathematics in secondary schools.

Nuffield website – with details about the research findings including summaries and more detailed reports.

Further work and excellent book: Key Ideas in Teaching Mathematics: Research-Based Guidance For Ages 9-19 then followed on from this original work (published 2013). Very relevant still.
Nuffield website – more information about background to research etc.

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This book has QR codes that link to many relevant curriculum resources.