As a newcomer to get_iplayer I have already a large collection of TV downloads that do not have subtitles. The downloads have exactly the same specifications as the downloaded via get_iplayer. Rather than going through the time consuming process of downloading all the videos again with subtitles is there a command that would allow me to search for the program (they are still online via a URL) and download the subtitle stream only?

You could do worse than read the documentation in wiki.

The example in documentation should have read: get_iplayer --subtitles-only 123 (no need for --get).

Indeed I could do worse! Worked a treat. Apologies for falling at the first hurdle! What a nicely stitched together program. Your efforts are greatly appreciated!