This study evaluates the efficacy of a mathematics intervention devised by Essex Educational Psychology Service (EPS), UK. The intervention was designed to develop understanding and skills across four key domains within arithmetical development, by applying the principles of errorless learning, distributed practice and teaching to mastery. A quasi-experimental design was used to investigate the success of the intervention in raising the level of arithmetical skills of lower achieving pupils up to National Curriculum Level 2. The results indicated that children engaging with the intervention made significantly more progress than children not engaging with the intervention, with ratio gains of 2.51 in arithmetical skills. In addition, gains were made in mathematical reasoning.