Thanks Paul.

My Poco goes back to default AAC the moment you step back from developer options, including after the reboot you suggested. I've tried this with Bluetooth switched on and off, phone paired and unpaired.

It's very strange - Poco support tell me my phone has aptX and other codecs but when I told them which headphones I'm using they said they wouldn't support this codec. No explanation of why not, clear as mud really! I've tried all the suggestions Google turns up too ...