S R Knight1, M Collie2, J Brush3, D Speake2. 1. Perth Royal Infirmary, Taymount Terrace, Perth, PH1 1NX, UK. stephenknight@doctors.org.uk. 2. Department of Colorectal Surgery, Western General Hospital, Crewe Road, Edinburgh, EH4 2XU, UK. 3. Department of Radiology, Western General Hospital, Crewe Road, Edinburgh, EH4 2XU, UK.