@mitu Hi mitu and thanks again!
I can confirm that P2 has the config file I posted earlier and also that I have no input overrides present.
I've exhausted all ideas and things to try from my end.
In the end I simply added the following line to my opt\retropie\configs\gb\retroarch.cfg file...
input_enable_hotkey_btn = "6"
This changes the hotkey from Select to another button on the righthand controller (lucky I have a spare).
This works. I can hit the new button (now hotkey) and Start and it will exit game back to ES.
But I'd still prefer to exit by pressing Select and Start!
What does this tell us?
Are we any closer to knowing what's happening here?
From my previous post... when the hotkey is the Select button, I can Reset game (Select + B) and I can Save state (Select + Right Shoulder) so Select does work as a hotkey.
It just doesn't work in combination with Start to enable me to exit game.
Thank you for any continued support.