Got it!
If there is a "yet to be recorded TV programme" in the folder, then you get taken inside the folder when you press OK. The PLAY / PAUSE etc buttons do not work when you play an episode.
If there is not a "yet to be recorded TV programme" in the folder, then you do not get taken inside the folder when you press OK, and a random? episode plays. The PLAY / PAUSE etc buttons do work when you play an episode.
Incidentally, I have discovered how to get back to the folder containing all the episodes when you are playing an episode and you don't know which it is.
While the recording is playing, get the program information by HOME > DOWN ARROW > LEFT ARROW > scroll left until you find the recording now playing > press OK. This takes you inside the folder and you can now choose which episode to play.
Can someone confirm please.