This was a vanilla mythbuntu system but I added KDE4 and uninstalled xfce.
I entered the usual system Administration->login screen->convenience. I
checked enable autologin, selected a user and 'applied'.

However, the system refuses to autologin. Any clues as to how to overcome this will be very welcome.

Can you make sure your system is using KDM as the display manager?
Kubuntu at least uses LightDM, and I think Xfce based derivates may be using GDM.