1. Centre for Rehabilitation Sciences, School of Health and Social Care, University of Teesside, Parkside West, Middlesbrough, Tees Valley, TS1 3BA, UK,
2. Hull York Medical School, University of Hull, Cottingham Road, Hull HU6 7RX, UK,
3. Department of Health Sciences, Area 3, Seebohm Rowntree Building, University of York, Heslington, York YO10 5DD, UK, lesley.gillespie@yahoo. co.nz
4. Evidence for Population Health Unit, Division of Epidemiology and Health Sciences, University of Manchester, 2nd Floor, Stopford Building, Oxford Road, Manchester M13 9PT, UK, rajan.madhok@ manchester.ac.uk