Virtualmin does not perform system updates on it’s own, by default – a lot of sysadmins prefer to do it themselves in case there is a problem.
You can configure how that’s handled in System Information -> Package Updates -> Virtualmin Package Updates.
As far as the SA updates go – Virtualmin relies on the distro-provided packages for all that, so if the distro provided scripts to pull down updates are disabled, it’s likely you’ll need to enable that to get updates.
Apologies for being such a pain asking dumb questions.
As I have said elsewhere, I have the GPL version installed on another (production) box and I managed to mess it up horribly by doing something wrong. That box is still limping along and will have to go soon. I am therefore determined that this time I will be very careful.