The Cognitive Acceleration (or Let's Think) approach to mathematics teaching is a Piagetian programme drawing on Vygotsky's research, developed at King's College London over 30 years ago, along with its associated professional development (PD) programme. This project sought to replicate the original studies conducted 10-15 years earlier and before much national curriculum change, through a professional development project with 41 teachers of children aged 6-12 from London in 2014. Results of pre- and post-test of mathematics attainment are reported for 232 students. Despite a shorter duration, data shows increased teacher efficacy, improved teaching and a mean gain equivalent to 2.6 months learning for benefitting students, which broadly mirrors cognitive effects of original trials with twice the duration. This evidence corroborates the impact demonstrated in several original CA research papers, and additionally details specific impact on teacher confidence and classroom practice.