Eventually I took the long route and recreated the style reference by pointing to an existing style in the path I required.
This necessitated that I copy the .css file to the network location, use the navigator, change the drive assignment to a UNC and then save it.
I then made it default.
And discovered three traps;
. unlike other parts of EG where "Delete" simply removes a reference such as a table or code object in a project, this physically deletes the style sheet file referenced in the style browser. Oops.
. if you type the path incorrectly, tough. "Go to gaol, go directly to gaol, do not pass go, do not collect 200 pounds"!!
. if you decide you don't like the style, you can't edit the version in the UNC path, but need to keep another copy that you edit and copy over the existing one.
However, I still can't get past the warning when I run the code on the server that says "style ... not found. Default style will be used instead." I also note in the ODS HTML reference that the URL for the style sheet is wrong, since the "file:" assertion is followed by a path using forward slashes!
Kind regards
David