Success! I don't login specifically, the PC starts up and goes to the desktop.
I confirmed my Username by Cntrl-Alt-Del - Task Manager - Users (and see only my Name).
That's put in Services-Seviio-Propoerties-Logon-This Account. (Although it's added ".\" ahead of my name and the password asterisks).
I stop and restart Serviio, checking at this point Windows can see everything on the network.
I open the serviio console and Library-Shared Folders. There's no longer the error showing on the UNC path ( I didn't know what that was
until now so was glad you made it obvious!) I could also paste that "UNC" directly into Explorer and see the folder.
And all the files appear on the TV! Thanks very much for your assistance

Can I see the filenames in Serviio console anyhow (or not) ?