When it came up again the boot screen appeared to think it was running 14.04 (rather than the previous 13.10) however at the login screen neither keyboard nor mouse got any reaction (and the ssh server wasn't running the machine hadn't registered with the router). I tried re-booting a couple of times with no effect.
I then broke into GRUB (at boot - press down the shift key) and booted the minimal 13.10 install (still there!) and ran the dkpg fix option (I previously tried minimal with 14.04 which again failed) which ran to completion. A reboot then gave me a working system!
Looking at the upgrade log file the last lines of /var/log/dist-upgrade/term.log read as follows:
Setting up openssh-server (1:6.6p1-2ubuntu2) ...Yes it stops there! I had it down as a problem with the NVIDIA graphics driver, this might suggest something else - I wasn't connected via ssh when I was running the upgrade so it wasn't a a case of the connection dying. Still puzzled but putting this here in case it helps anyone else.
Installing new version of config file /etc/init/ssh.conf ...
Installing new version of config file /etc/pam.d/sshd ...
ssh start/running, proce
The other problem I found (so far!) is that Ctrl-Space doesn't appear to do anything - I use this key combo a lot - in Emacs. So it was a bit painful, a websearch reveled that ibus was the problem so I ran ibus-setup and stopped that grabbing Ctrl-Space for 'next input method' and assigned it to a different key.
So far everything else appears to work...