Heather Wardle1, Stephanie Bramley2, Caroline Norrie3, Jill Manthorpe3. 1. London School of Hygiene and Tropical Medicine, 15-17 Tavistock Place, London WC1H 9SH, United Kingdom. 2. NIHR Health and Social Care Workforce Research Unit, The Policy Institute at King's, King's College London, Strand, London WC2R 2LS, United Kingdom. Electronic address: Stephanie.bramley@kcl.ac.uk. 3. NIHR Health and Social Care Workforce Research Unit, The Policy Institute at King's, King's College London, Strand, London WC2R 2LS, United Kingdom.