In the end I tried tapping both shift keys really fast on a system boot. This would only bring up the menu about one time in five. I think it's because the mohterboard is so new and so very fast at getting to the bootloader stage.
Then I chose to go into rescue mode and then from the menu when to a root prompt.
After getting a root prompt I did the usual..
# mount -o remount,rw
Once the file system is in rw mode I then edited to the sudoers file.
- Always use visudo to edit your sudoers file so that syntax is checked for you before saving.
# sudo visudo
- Annoying, ubuntu uses nano as it's default text editor, which put me off using visudo. So another good tip is to change your default editor by using update-alternatives
# sudo update-alternatives --config editor
- It's probably a good idea to set a password (a very secure one obviously) for root.