Whist inclusion is recommended for most children most of the time it remains difficult to implement. In this paper, we present the process undertaken to review and redesign a pre-existing complex intervention (The CIRCLE Framework) which was designed to enhance teachers confidence and competence in provision of universal first level supports for 5–12 year old children with additional support needs. The approach presented draws on the Medical Research Council guidance for the development of complex interventions. A series of ten co-design workshops with 70 stakeholders was completed, applying interactive and participatory methods. Analysing outputs of each workshop revealed recurring design ideas that became the main aspects of the new framework and associated manuals. Intervention content, theoretical frameworks, manuals to support use in practice and implementation strategies were developed. On completion, the updated intervention was extended up to 18 years of age and redistributed to all teachers in the participating local authority. We present the main conclusions and interpretations around the design and naturalistic implementation of the framework, and reflections on use in practice, including a detailed list of recommendations for implementation across schools and staff.