Laugh,
I checked the error.log file. I should check it more often, but I can't see anything related to this issue.
Michael,
I am using the standard Professional theme, except for the header.thtml and footer.thtml files have been replaced with ones that match the rest of my site. Unless something in the header and footer could affect the config, that shouldn't be the problem. When I did the clean install on this new server it should have had all the original standard files. I didn't leave anything out, but afterwards just copied across the previous header and footer from the old site on the old server that was working.
Greg.