I bought a new AP.
Started the UniFi controller, which reported my APs as needing updating.
Updated them, seemingly OK.
Connected the new AP to the network, needed updating from something really old.
New AP then showed as Disconnected / Needs updating, and try as I might it didn't work.
Returned to supplier for refund.
Turned off my existing AP here to move it. On powering it up, the same symptom - disconnected, unpingable. WiFi side still working, but clients cannot get a DHCP address. I will try TFTP again, with little hope.
I have disabled automatic firmware upgrades in the UniFi controller, and reverted my remote AP to 18.104.22.16832 so it should still work after being powered off.
Meanwhile, bought some BT White Disks as I really need my WiFi network!