Purpose: The quality of voluntary sector-led community health programmes is an important concern for service users, providers and commissioners. Research on the fidelity of programme implementation offers a basis for assessing and further enhancing practice. The purpose of this paper is to report on the fidelity assessment of Living Well Taking Control (LWTC)--a voluntary sector-led, community-based education programme in England focussing on the prevention and management of type 2 diabetes. Design/methodology/approach: This fidelity of implementation (FoI) study was conducted with the Devon-based LWTC programme. A fidelity checklist was developed to analyse audio records of group-based lifestyle education sessions--implementation was rated in terms of adherence to protocol and competence in delivery; the influence of wider contextual factors was also assessed. Kappa statistics (?) were used to test for inter-rater agreement. Course satisfaction data were used as a supplementary indicator of facilitator competence. Findings: Analysis of 28 sessions, from five diabetes prevention and two diabetes management groups (total participants, n = 49), yielded an overall implementation fidelity score of 77.3 per cent for adherence (moderate inter-rater agreement, ? = 0.60) and 95.1 per cent for competence (good inter-rater agreement, ? = 0.71). The diabetes prevention groups consistently achieved higher adherence scores than the diabetes management groups. Facilitator competence was supported by high participant satisfaction ratings. Originality/value: An appropriate level of implementation fidelity was delivered for the LWTC group-based education programme, which provides some confidence that outcomes from the programme reflected intervention effectiveness. This study demonstrates the viability of assessing the FoI in a voluntary sector-led public health initiative and the potential of this method for assuring quality and informing service development.