ok so spent some time figuring this out, mostly just looking at what i might have messed up. not totally certain but i believe at somepoint I edited /var/lib/bluetooth/(devid)/linkkeys becuase I'd had trouble with bluetooth keyboard.
in that edit I managed to leave a blank line at the start of the file.
I removed that blank line and rebooted. all now seems well again.
important lesson there about not messing with stuff at a low level unless you're prepared to break it :-)