Funding for this research was provided by:
Medical Research Council (MR/K024973/1, MC_UU_12019/1, MC_UU_12019/4)
Received: 15 January 2019
Accepted: 16 June 2019
First Online: 25 June 2019
Ethics approval and consent to participate
: Relevant ethics approval has been granted for each data collection; ethical approval for the most recent assessment in 2014–2015 was obtained from the Queen Square Research Ethics Committee (14/LO/1073) and the Scotland A Research Ethics Committee (14/SS/1009). Study participants provided written informed consent.
: Not applicable
: The authors declare that they have no competing interests.