1. MRC Epidemiology Unit, University of Cambridge, Cambridge, UK
2. School of Clinical Medicine, University of Cambridge, Cambridge, UK
3. Fakenham Medical Practice, Norfolk, UK
4. Norwich Medical School, University of East Anglia, Norwich, UK
5. Division of Psychological & Social Medicine, School of Medicine, University of Leeds, Leeds, UK
6. Department of Medicine, University College London, London, UK
7. Primary Care Unit, Department of Public Health and Primary Care, University of Cambridge, Cambridge, UK