1. James Cook University Hospital, Marton Road, Middlesborough TS1 3BW, UK.
2. London School of Hygiene and Tropical Medicine, Clinical Effectiveness Unit, The Royal College of Surgeons of England, 35-43 Lincoln’s Inn Fields, London WC2A 3PE, UK.
3. Health Services Research Unit, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.