It may not be a front/back focus issue Mark which is why I made reference to this. Out of interest when you tested did you put the lens out of focus before allowing the camera to re focus and take your test shot.
If you have any concerns do as I did get Canon to check it out under warranty. You will be without your camera for a week but they will thoroughly test it to ensure there are no issues.
I got mine back to Elstree but there is an approved Canon repairer in Colchester if you are ever down that way.
Colchester Camera Repair Services
Professional Camera, Lens, Professional Lens, Flash, Analogue Compact Camera, Digital Compact Camera, Analogue SLR Camera, Digital SLR Camera
6 Kings Court
Off Newcomen Way
Severalls Business Park
Colchester
CO4 9RA
Tel: 01206 843 322
Fax: 01206 843 323
Web: http://www.camera-repair.co.uk
Probably best though if you are still unhappy after testing to see what WE have to say