Thank you for your help. I have made a step in the right direction.
I applied this "fix" and can now connect my bluetooth headset. Solved?
Well, it says it is connected, however I did not get the "Bluetooth connected" voice in my headset that I would normally get.
The issue now remains that even when connected, the headset does not appear as an audio device for me to select as the output device.
Managed to get some audio through the bluetooth headset, but only one ear and very jittery and unstable. Worse than last year when I first tried all this.
Now back to not being able to connect. (I wish there was a pattern to this intermittent connection issues).
My machine is 3 years old. Should be fine.
I can't try your suggestion - the BT headphones are not listed as a sound output device.
I have managed to get my BT headphones to connect to Zorin OS installed on another machine (even older).
Is there a way I can compare the settings between these two machines?
I have looked at some folders in /etc and tried to compare them but so far have not found anything amiss. Where else are bluetooth / audio settings saved?
I have managed to resolve this issue, but not convinced it is the "right way".
I had tried just about everything, and through a combination of changes now have bluetooth audio now working through my Sony BT headphones.
I disabled the onboard BT module in my HP Elitedesk BIOS.
I plugged in a generic USB BT receiver.
I re-installed all packages refering to bluetooth
I used bluetoothctl and entered the command for "set default"
I did run across other issues which have now resolved themselves. These included what must have been conflisst with my BT keyboard + mouses's unifying receiver.
The headset now connects and works as expected, without jitter. It is still slow to be "found / connected" but I'm not going to pursue this further in case I break it.
So my problem / solution seems to revolve around incompatibility between the onboard BT and my headset, which I have resolved by using a 3rd party USB BT dongle.