[1] Univ Bristol, Sch Social & Community Med, Ctr Acad Primary Care, Canynge Hall,39 Whatley Rd, Bristol BS8 2PS, Avon, England
[2] Univ Sheffield, Sch Hlth & Related Res ScHARR, Clin Trials Res Unit, Sheffield, S Yorkshire, England
[3] Univ Bristol, Sch Social & Community Med, BRTC, Bristol, Avon, England
[4] Univ Nottingham, Queens Med Ctr, Fac Med & Hlth Sci, Nottingham Clin Trials Unit, Nottingham, England