Thank you all for the advice regarding the Xserver problems. I am still not sure what happened here but following your advice and with some other ideas this worked.
Disable nouveau when booting press e add nouveau.modeset=0 press F10. This boots and allows me to log in.
ps -A | grep shows that startx, xinit and Xorg are not running so clearly a step in the right direction.
startx fails with no screens found and that it is using /etc/X11/xorg.conf.
Renaming /etc/X11/xorg.conf to /etc/X11/xorg.conf.bak and trying again also fails with no screens found but now reports that it is using /usr/share/X11/xorg.conf.d
A quick look in there shows a file 20-nouveau.conf which identifies the video card as Nvidia and uses the nouveau driver. Renaming that to 20-nouveau.conf.bak and restarting the system brings everything up normally.
I have no idea what happened, what was wrong with these files or why renaming them worked. In fact not even sure what video drivers I am running right now. Much still to investigate.
Thanks for the suggestions.
Stripes