I tried all methods presented by the generous members of this list, but with no good fortune. I hope someone will find it useful.
Use a Ubuntu live distribution CD to reach a command-line terminal. First check that your $HOME directory (/home/username) has appropriate permissions, and that all directories can be examined with the "ls" command.
Then use
Code:
sudo apt-get remove nautilus
and remove any nautilus files and folders which might remain in $HOME. Also remove .ICEauthority from /var/lib/gdm (if it exists). Then you can reboot with
Code:
sudo shutdown -r now
and on restart, get to the CLI and use
Code:
sudo apt-get install nautilus
sudo shutdown -r now
and remove the CD from its drive. On reboot, I can only hope that you have the same luck as me, and arrive safely at your usual login form.
Thanks to all, and best wishes,
Phil