Maybe try a plex dance if you have not all ready.
I have this show myself and the episode is named as follows Space: 1999 (1975) {tmdb-134}/Season 01/Space: 1999 - S01E09 - Mission of the Darians.mp4 and its detected by plex.
I am using tmdb id for matching but tvdb id should work just fine so I believe a plex dance will sort it out for you.
