I'm not sure what I did, but now my KDE desktop is just a blank screen. I am still able to log onto a gnome session, and the guest account has a perfeclty fine Kubuntu session, but no matter what I do I can't get a Kubuntu session with my own login.
Issue resolved at https://bugs.kde.org/show_bug.cgi?id=347919
Somehow ??? permissions had changed, in .cache a file .cache/.ksycoca5 was owned by root
The following command worked from the terminal:
sudo chown yourusername:yourusername ~/.cache/ksycoca5
it all came back.
In the interim I installed gnome.