I wrote that post “too soon”… it’s not quite solved…
Bit of a kludge / workaround…
Have to put the headset into Pairing Mode (keep holding power button down when powering them up) - then go into the Bluetooth “settings” and if they’ve already been paired, click on “Connect” several times till they connect - then they’re in Stereo mode… otherwise get choppy playback in mono only…
Just tested this on Ubuntu 18.10 on my work laptop - and the "kludge "works…
I think installing blueman and running “pulseaudio --k” were probably unnecessary and pointless, and it kinda looks like I’ve may have sorta “broken” bluetooth on my 18.04 laptop at home (few anomalies - and it created an additional app-indicator for Bluetooth - as well as the default one you get with Bionic)… Might just wipe it anyway and install a fresh 18.10, ready to go rolling release then upgrade to 19.04…