[1] Univ London London Sch Hyg & Trop Med, Dept Publ Hlth & Policy, Hlth Serv Res Unit, London WC1E 7HT, England
[2] Kensington & Chelsea & Westminster Hlth Author, Dept Publ Hlth, London, England
[3] Royal Free & UCL, Dept Primary Care & Populat Sci, London, England
[4] Univ Southampton, Wessex Inst Hlth Res & Dev, Southampton, Hants, England
[5] Charing Cross Hosp, Dept Renal Med, London, England
[6] Univ London St Georges Hosp, Sch Med, Dept Publ Hlth Sci, London, England