[1] Univ Hertfordshire, Ctr Res Primary & Community Care, Hatfield AL10 9AB, Herts, England
[2] Univ York, Social Policy Res Unit, York YO10 5DD, N Yorkshire, England
[3] UCL Med Sch, Res Dept Primary Care & Populat Hlth, London, England
[4] Univ Cambridge, Dept Publ Hlth & Primary Care, Cambridge, England
[5] Newcastle Univ, Inst Ageing & Hlth, Newcastle Upon Tyne NE1 7RU, Tyne & Wear, England