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Contributors JM and CB critically assessed the study, and drafted and agreed on the final manuscript.
Funding This article presents independent research funded by the National Institute for Health Research (NIHR) under its Programme Grants for Applied Research Programme (Reference Number RP-PG-1210-12003). The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health.
Competing interests None declared.
Provenance and peer review Commissioned; internally peer reviewed.
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