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.