Thanks, Dave, for your input. I'm afraid I can't agree with you that PDVD 19 has got it right at all. To me it makes perfect sense to have subtitle files with precisely the same name as the video file (there is an obvious 1-to-1 correspondence) and for the video player to recognize this correspondence and select the subtitle file at play time automatically. It seems obvious to me, also, that should there be no corresponding subtitle file then the video player ignores all subtitle files it finds. I think earlier versions of PDVD (16, 17?) used to do this.
Your response suggests that there is no way of doing what I asked - automatic selection of subtitle file and ignoring all subtitle files where there is no match. I'm considering either finding another video player that uses straightforward logic in handling subtitle files, or using a video editor that allows me to embed the subtitle file within the video for playing in PDVD. It seems to me that all PDVD needs to do is ignore all subtitle files where there is no match.
Another problem I have found is that sometimes the subtitle facility in PDVD selects "SubtitleHandler - [English]" and displays all dialog as subtitles. This is a considerable nuisance as one needs to turn off subtitles for this video, necessitating turning them on again for the next video that has a subtitle file.
Paul.