I'm please to say we have progress.
Fresh VM (This is using bigv.io by the way)
Checked and it does come with mysql installed, not running, but installed.
It was this installation that had stuck the /etc/my.cnf with the skip-networking option.
So all I've done is comment out that skip-networking line, and tried the
iRedMail installation again.
It managed to complete with no errors and after a reboot I have a much happier
looking roundcube logon page sitting at /mail
I haven't tried logging in yet as I don't have any users definied. But everything looks
much, much happier.
Thank you