Thanks Otto, but I’m still not there.
The checkbox ‘use embedded metatags’ is ticked.
I moved the offending folder out of the sight of Plex, scanned the library (automatically), emptied the trash, double checked the naming scheme, moved the folder/files back and (automatically) re-scanned the library. Nothing has changed.
My Finder folder has been set as I want it and shows Media/Music /Bach, Johann Sebastian/Cello Suites, BWV 1007-101 - Pierre Fournier/track…
The metadata in Musicbrainz has been changed to reflect this, i.e. Artist = Johan Sebastian Bach; Album = Cello Suites, BWV 1007-101 - Pierre Fournier; Album Artist Sort Order = Bach, Johann Sebastian.
Weirdly (to my eyes anyway), Plex shows Artist = Cello Suites, BWV 1007-101; and the Sort Artist = Bach, Johann Sebastian. So I guess, I’m halfway there. 
Also, I note that when doing the Plex Dance the Trash is empty despite the fact that I have jus re-scanned the Music section of Plex and it has removed the offending piece of music.
Appreciate your help.
