@Midnight_Voice I suspect it is because the players eg IPlayer/4 do not recognise the More button function whilst they do the Delete/Subtitle button function.
When watching EPG channels....When you do use More button on the Pro Remote you may have to navigate left/right and you will need to press a further OK to effect switching subtitles on/off.
Thank you for the comments. I am more than happy to use the old remote with the Delete/Sub button along with the Pro box. It's a one click solution as opposed to the tortuous alternative.
Incidentally after talking to a BT Guide, I used the new remote to navigate from the Home button across to the Settings cog, scrolled down to Accessibility, selected Subtitles on, then Done. Result - no subtitles!!
With the old remote it's on/off at the touch of a button.
Case closed.
It looks like maybe Up Arrow on the Pro remote (and even the older remotes) may be a common thread for reaching subtitles?
That may be the case, but after that the procedure seems to vary depending on the app.
Hard to be sure about things when some programmes don't seem to support subs so you don't know whether it's the box, the remote or the programme you're watching. I've given up experimenting and will rely on the good old DEL/SUB remote and hope it will continue to work.