Developing and supporting implementation of a dialogic pedagogy in primary schools in England

•We evaluate the implementation of a dialogic teaching intervention and its impact on pedagogical practices.•Students were on average two months ahead of control peers in English, mathematics and science.•Teachers receiving the intervention made significantly greater use of dialogue and discussion.•Student contributions following the intervention were more extended and expansive in content and reasoning.•Stimulated critical reflection using video played an important role in the process of pedagogical change.