Following user: "coteyr's" suggestions (5.5.13) 'sudo X :2' did give me a black screen but no "X" for the mouse pointer. The log for this is apparently /var/log/X.2.log, but this did not show any errors. Also had to ctrl-alt-del out of this.
I then created a new user and could login using it, but it would not log out. I could also look into the old-user directories down about 5 levels but had no rights to look at any files. If I read your logic correctly this all means that xorg and lightdm are both ok and that it is a problem with my old user configuration.
Performed logical testing procedure thanks to coteyr and followed his advice step by step to find the personal config was "borked" for unknown reasons. Instead of deleting .config I did as suggested and typed: mv .config .config.old which I gather backs it up and allows a new .config to be created next time you boot/login. This allowed login of my original user and hence direct access to all my old files. However, there were a few "12.04 has experienced an error" windows popping up to do with: /usr/bin/gnome-session /usr/bin/gsettings /usr/bin/l2tpIPsecVpn all crashed. So in the end this was not a lightdm or xorg problem, but a unity/gnome config problem. 7th May: Since the second login on 12.04 I have not been able to get online, but have just been able to check using wifi, which works. So my DSL connection is now not recognised. I know this is not the same question but can anyone help or should I post this as an additional problem?