OK, so I cured it by plugging in the headset mains charger and immediately unplugging it again. Presumably this does a reset on the headset. Hooray!

However, prompted by Benson's message, I tried plugging in the ear buds/mic. If the headset is paired, unplugging the earbuds consistently does not return audio to the built in speakers. It is getting very confused. But since the headset is working, it goes back to the headset in those circumstances.

Anyway, I'm back up and working again. Thank you both.