1. College of Social Sciences, University of Glasgow , Glasgow G12 8QQ , UK
2. Department of Health Services Research and Policy, London School of Hygiene and Tropical Medicine , 15-17 Tavistock Place, London WC1H 9SH , UK
3. School of Geography and the Environment, University of Oxford, South Parks Road , Oxford OX1 3QY , UK
4. School of Geography and the Environment, University of Oxford , South Parks Road, Oxford OX1 3QY , UK