I think I might have acidentally found the solution.
In the Menu system that runs the whole site, because not even bog standard HTML files would work either, I had to replace all the forward slashes in paths to a back slash.
Because IIS is setup to use localhost, you apparently don't have to put the actual pathnames in because if you look in the IIS services, all the folders and files for my web site are shown to be directly under Default Web Site.
So, I guess it's no wonder nothing worked?
Terry