1. Medical Research Council; Social and Public Health Sciences Unit; 4 Lilybank Gardens Glasgow UK G12 8RZ
2. Mid Sweden University; Department of Health Sciences; Östersund Sweden S-831 25
3. London School of Hygiene and Tropical Medicine; Department of Social & Environmental Health Research, Faculty of Public Health & Policy; 15-17 Tavistock Place London UK WC1H 9SH