1. National Health Primary Care Trust Westminster, 15 Marylebone Road, London NW1 5JD, UK
2. Department of Epidemiology and Public Health, University College London, 1-19 Torrington Place, London WC1E 6BT, UK
3. Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK