BC, LEG and DL have no competing interests. SW is a non-executive director at NHS-England. KK is the deputy director and part funded by the NIHR Policy Research Unit in Health and Social Care Workforce. SAMS is supported by the National Institute for Health and Care Research (NIHR), Maudsley Biomedical Research Centre at South London and Maudsley NHS Foundation Trust and the National Institute for Health and Care Research NIHR Advanced Fellowship (Dr Sharon Stevelink NIHR300592).