Just a further update on this one. I have an idea that this may be connected with a failure in the custom fields bulk upload function. I uploaded 700+ rows which reported 'success' (though Jerry Asher's caution in the remarks was to treat that with suspicion). Everything seemed ok but it has since transpired that only the first 250 were uploaded, for the rest of the products in the system the custom fields remained null.
I suspect therefore that this error is thrown by the .tcl script that generates the confirm changes page when it tries to display the custom fields for the product whose record is in the process of being updated by edit-2.tcl. Where we have been uploading photos, these are correctly uploaded and placed in the file system, but the confirm page chokes.
I will be trying to tie down the reason for the part failed bulk upload this weekend, if anyone has any ideas meanwhile I'd love to hear them.
Regards
Richard