In the current study we will test whether relationships with Peer Navigators can improve mental health and quality of life, compared to people in similar services who do not have a Peer Navigator. Peer Navigators will be based in services run by The Salvation Army. We will invite people who are experiencing homelessness and problem substance use from services based in 20 cities and towns in England and Scotland to participate. These services will have an equal chance to either deliver the 12-month long SHARPS intervention, or the usual support on offer. Peer Navigators will provide practical and emotional support to 25 clients for up to 12 months to help them to make changes in their lives. We will compare people’s mental health/quality of life in the services that had the Peer Navigators with those that had usual support. We will measure mental and physical health, quality of life, alcohol and other drug use, access to social support, and ability to participate in society, when people join the study, and again 6, 12 and 15 months later. To understand how the intervention works in practice we will run a process evaluation. We will interview a sample of participants who had a Peer Navigator at the end of the intervention and three months after. We will also interview the Peer Navigators, and a sample of wider staff members. We will conduct a survey with staff/Peer Navigators and observe all services across the study.