Virtualmin appears to set up the default user for ftp access OK, and it reports in the domain - users - description, that ftp login is enabled (ftp login? enabled = YES).
However, when actually trying to login with ftp it fails.
Ticking the user selection box on and then "modifying" the settings to enabled for ftp and saving does in fact enable ftp login for the default user to occur.
Still a newbie here but I am pretty sure this is a small bug as I have had this on most of the domains I have created now.