Thanks for the advice you've given so far. I think the BTRFS tools are needed because my systemd-container is built on /var/lib/machines which is BTRFS and it tries to make subvolume snapshots.
I got to the bottom of my password issue. I had winbind installed and it prefixed the username with the Windows Domain of my machine. I purged winbind and I was able to su from a non-root account, give the password I'd entered and access the admin account. I then went to plinth and was able to create a new user. So that's that fixed! Thanks for the help. Walter
