I think the issue was cached images from Vray. Once I had moved the original images everything worked as expected. Occasionally I had to resize the image texture to match the original, but all sorted.
OK it seems thomthom's plugin 'Cleanup' does the job in a round about sort of way, not to mention a few other extremely useful things.
http://forums.sketchucation.com/viewtopic.php?t=22920