Cheers Walter.
None of the above were working which was getting worrying.
I managed to solve it by plugging an additional display into the hdmi port. There appeared my Premiere desktop. Once in there I could detach the thunderbolt, open preferences (and add a hotkey for accessing this quickly) before resetting the playback options.
Managed to get things working again now. Phew – had a client coming in for a sound mix/colouring session which was going to look embarrassing with nothing working!