So today I forgot the root password to an Ubuntu VM I was playing with recently. Did the normal thing  and hit escape at the grub boot screen and dropped to a root shell followed by the command to reset the password on the account..
passwd <account name>
and was greeted with:
passwd: Authentication token manipulation error
passwd: password unchanged
So with a bit of help from google  it turns out the drive just needs to be mounted read/write:
mount -rw -o remount /
before running the passwd command as before.