Well, after a fair amount of hassle, I've worked out how to get root access to the hard disk on the system I have installed xubuntu 11.10 but the main account password is broken.
You boot the "Live CD" system and you can then mount the disk and use sudo to become root.
So, I can edit /etc/passwd and I can edit /etc/shadow but removing the password field from my main/default user in the /etc/shadow file *still* doesn't allow me to login. Grrrrr!!!
What are the requisites for a login to have no password? I thought it was a 'x' in the password field in /etc/passwd and nothing in the password field in /etc/shadow but that seems not to work.