Can't change software sources via GUI following upgrade to 17.10 (using Gnome)

[Colin Law]

Having upgraded to 17.10 (from 17.04), when I open Software and
Updates I am unable to select or deselect s/w sources. I do not get
the popup asking me to enter my password.  I do not see the problem on
another machine with a fresh install of 17.10.

Any suggestions where I might look to sort it? No success searching
Google and Launchpad.

Colin