Reverting to 25 fps
I was tinkering with some quality settings, using the commands (in this order):
get_iplayer --prefs-add --tvmode=best
get_iplayer --prefs-add --tvmode=best  --fps50

…and don't know how to undo the latter.

For the same TV series, my downloads are 2GB instead of 1GB, and using Get Info on the files I can't see any difference (pre- and post-commands they're all 1280 x 720 H.265 AAC) other than after running these commands, there's a new line in the Properties that says "Color profile: HD (1-1-1)"

Is it the 50 fps setting that's the likely culprit? And if so, how do I revert back to 25 fps please? I tried running
get_iplayer --prefs-add --tvmode=best
again, and that's not helped. I also tried
get_iplayer --prefs-add --tvmode=best  --fps25
but that's not a thing.

get_iplayer --prefs-del --fps50

The list of commands available to get_iplayer is covered in the Options wiki entry. Here's the link the the section covering config options - where the --prefs options are listed:
That's brilliant, thank you for the help.