The 3S-YP study is being co-sponsored by King’s College London and South London and Maudsley NHS Foundation Trust . This work is supported by the Medical Research Council and Medical Research Foundation [grant number MR/S020365/1]. This research is also supported by the NIHR Maudsley Biomedical Research Centre (BRC) at the South London & Maudsley NHS Foundation Trust and King’s College London. The views expressed are those of the author(s) and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care.