Hello Paul,
Thank you.. I have got it to work following your advice. I think I have also identified the source of the problem.
I performed a factory reset on the speaker - not just a standard reset (which doesn't clear pairings).
Switched on the B2 and cleared all paired items (settings menu on the B2 app).
Re-paired the speaker and the B2.
Once paired played music from the B2 through my Hi-Fi... All OK.
Switched on the speaker - as soon as it automatically paired, the B2 went haywire again..
Went through the full reset procedure as above for both speaker and B2 but this time unchecked the box in Settings/Bluetooth that says "Bluetooth and speakers together".
Success. Played through the Hi-Fi OK then once I had switched on the speaker, output transferred to the speaker, cutting out the Hi-Fi output. Switching off the speaker sent output back to the Hi-Fi..
It seems that the problem may lie with the B2's ability to direct output both to external Hi-Fi speakers and Bluetooth together. It doesn't work.. Not in my case anyway.
As far as I am concerned, the situation is acceptable as I am highly unlikely to want to play both Hi-Fi and bluetooth speaker together but it may be something that should be looked at for future software upgrades.
Thank you for your help and for the comments by other contributors..
Pat.