The United Kingdom has committed to eliminating viral hepatitis as a public health threat. Innovative interventions for marginalised populations are required to realise this goal. In 2016, the HepCATT study team implemented a complex hepatitis C (HCV) intervention in three English drug treatment services, with five controls. We report qualitative study findings from two intervention sites to explore intervention success and transferability potential. The intervention comprised multiple components, including a nurse facilitator, peer support and education initiatives. Qualitative data were generated at baseline (2014) and post-intervention (2016) at two sites through in-depth interviews, focus groups and observations. The 96 participants comprised drug service and intervention providers and clients with an injecting history. Data were triangulated and thematically analysed. Client engagement with a HCV treatment service rose from 16 at baseline to 147 in 2016. There was no comparable increase at the five control sites. Baseline testing and treatment barriers included: limited HCV knowledge; fear of diagnosis and treatment; precarious living circumstances and service-specific obstacles. Treatment engagement was aided by: intervention timeliness; improved communication structures; personalised care; streamlined testing and treatment pathways; peer support. Multiple interrelated components influenced the increased levels of treatment engagement documented in HepCATT. The nurse facilitator, involved in implementation and innovation, was key to intervention success. Baseline barriers correspond with international literature - indicating transferability potential. Control data indicate that biomedical innovation alone is not sufficient to increase engagement amongst the most marginalised. Sustainable resourcing of community services is crucial to effect change. This article is protected by copyright. All rights reserved.