ObjectivesStakeholder co-production in design of public health programmes may reduce the ‘implementation gap’ but can be time-consuming and costly. Prototyping, iterative refining relevant to delivery context, offers a potential solution. This evaluation explored implementation and lessons learnt for a 12-week referral-based weight-management programme, ‘Momenta’, along with feasibility of an iterative prototyping evaluation framework.DesignMixed methods evaluation: Qualitative implementation exploration with referrers and service users; preliminary analysis of anonymised quantitative service data (12 and 52 weeks).SettingTwo leisure centres in Northumberland, North East England.ParticipantsIndividual interviews with referring professionals (n=5) and focus groups with service users (n=13). Individuals (n=182) referred by healthcare professionals (quantitative data).InterventionsThree 12-week programme iterations: Momenta (n=59), Momenta-Fitness membership (n=58) and Fitness membership only (n=65).Primary and secondary outcome measuresPrimary outcome: Qualitative themes developed through stakeholder-engagement. Secondary outcomes included preliminary exploration of recruitment, uptake, retention, and changes in weight, body mass index, waist circumference and psychological well-being.ResultsService users reported positive experiences of Momenta. Implementation gaps were revealed around the referral process and practitioner knowledge. Prototyping enabled iterative refinements such as broadening inclusion criteria. Uptake and 12-week retention were higher for Momenta (84.7%, 45.8%) and Momenta-Fitness (93.1%, 60.3%) versus Fitness only (75.4%, 24.6%). Exploration of other preliminary outcomes (completers only) suggested potential for within-group weight loss and increased psychological well-being for Momenta and Momenta-Fitness at 12 weeks. 52 week follow-up data were limited (32%, 33% and 6% retention for those who started Momenta, Momenta-Fitness and Fitness, respectively) but suggested potential weight loss maintenance for Momenta-Fitness.ConclusionsIdentification of issues within the referral process enabled real-time iterative refinement, while lessons learnt may be of value for local implementation of ‘off-the-shelf’ weight management packages more generally. Our preliminary data for completers suggest Momenta may have potential for weight loss, particularly when offered with a fitness membership.