Peer advocacy and access to healthcare for people who are homeless in London, UK: a mixed method impact, economic and process evaluation protocol. - Rathod SD; Guise A; Annand PJ; Hosseini P; Williamson E; Miners A; Bowgett K; Burrows M; Aldridge RW; Luchenski S; Menezes D; Story A; et al. (BMJ open, 2021)