hello all, after much discussion Mifsuds agreed to take the lens back. They tested against another 100-400 they had in stock and confirmed the other version was 'much sharper' but refused to refund the return postage on the grounds that this was variability within Canon's products and not their fault. I disagree about the second part of that statement, if you are selling a E++ lens and don't both to test it against other lenses or to see if it is sharp throughout the range then I reckon you should pay the postage. They continue to disagree. I have since purchased the 70-300L from MPB photographic.
Thanks again for your invaluable assistance.
IQ falls on both the camera and lens, so it is impossible to really guarantee a lens is sharp for all cameras, due to numerous variables (body, person, etc). Any used equipment rating is really on its appearance as compared to new, and whether everything is operational.