[1] UCL Inst Child Hlth, Populat Policy & Practice Programme, 30 Guilford St, London WC1N 1EH, England
[2] London North West Hosp NHS Trust, Northwick Pk Hosp, Dept Paediat, London, England
[3] Nottingham Univ Hosp NHS Trust, Nottingham Childrens Hosp, Nottingham, England
[4] Univ Coll London Hosp Fdn Trust, Dept Paediat & Adolescent Med, London, England
[5] Great Ormond St Hosp Children NHS Fdn Trust, Dept Adolescent Med, London, England
[6] Barts Hlth NHS Trust, Newham Univ Hosp, London, England
[7] UCL, Inst Epidemiol & Hlth Care, London, England