@Mike_F:
Don't ask me why but I seem to have success!
I followed the instructions 'Connecting to your access point using firmware 1.19 or later' and downloaded the terminal program. Went through the process of SSID and Password and got the right response. I then went ahead with unplugging the USB and firing up the printer. Everything seems to be working just fine and it is reporting all the latest versions.
So, the big question is why did it hang when upgrading and never report it had succeeded?
Perhaps because you didn't do the step of running the M587 command, either from a terminal emulator or from a macro file as suggested in the upgrade instructions.
Future firmware updates will be easier. The update to 1.19 on Duet WiFi was special because we changed the way that the SSID and password are set up and stored.