[Klojum]

Please can you advise me how I ascertain the file protocol I am using to stream the movies?

Sorry, I must have overlooked the mentioning of the external drive. (I hear sun strokes can do that).

I downloaded Kodi from filehippo.com.

I know that site from way back when. I also thought that sometimes they packaged the installer file with their own, adding sponsored crap in them.
Next time, just download the Kodi app from our own download site.

It's possible that the subtitle add-ons are unable to store the downloaded subtitle onto the external harddrive.
Kodi's debug log (wiki) should be able to see that. If you can repeat the whole process and provide us the log file, we may see why subtitles aren't visible.