Thank you for your replies @raylock1 @Neville @WhiteHat
Phone and bluetooth are not causes of this problem. Beeps/changes were happening with bluetooth turned off for all devices. Beeps/changes happening with phone off and with phone on. I could connect HAs to the app and thereby I was able to change HA volume and program…but immediately the crazy beeps/changes messed with the settings again.
I brushed and wiped both switches carefully. Close inspection with magnifier did not reveal any debris around the switches.
Trying the HAs one at a time was the best advice, thank you. Left HA is the culprit. I will have to get it sent away for repair if possible. It might have a more complex fault than dirt in the switch. It did odd things while I was testing. E.g. a short while after I’d turned Left HA off and left it on the table, it spontaneously turned on, causing beeps/changes to start again in Right HA that I was wearing. Very weird.
Extremely disappointing to have a showstopper fault in expensive HA only 15 months old