Success! I have a working phone!

The problem was caused by the file
/etc/udev/rules.d/51-android.rules
being missing; it was not created during the installation of the UBPorts Installer software. I followed this link to manually create the missing file, and then manually add the udev rules. After that everything worked as expected.

http://docs.ubports.com/en/latest/userguide/install.html

I would expect the UBPorts Installer to check and throw an error if the udev rules are missing. I will register a bug against the installer.

Thanks to everyone for all your help.
Regards,
Barry T

@alter Thanks for the information. I have made a suggestion for a change to the hash symbol. We shall have to wait and see if anything happens.

@lduboeuf Thanks for the reply.

16.04 (OTA-24), and stable channel.

It's going from bad to worse:-
I tried to re-install just the Gallery app, so I used the App Store to remove the Gallery app but now it refuses to re-install.
I have installed an equivalent photo management app called Imaginario which works correctly.
There is definitely something wrong with the Gallery app.