After spending 20 minutes watching top - I discovered a process called auto.pl started using masses of memory and than was overtake by a process called kswapd0 - which I found out is a system process trying to free up resources for the auto.pl process. Between the two of them they quickly crashed by box.
Auto.pl is Usermin process - I installed a cert yesterday after Lets Encrypt failed ( this was also why I updated my box) I saw the virtual server’s main account had 300k emails and as their was no empty folder option I enabled auto clean and gave it no more thought.
My server was crashing every time the cron ran trying to empty this mail box…
The lesson I have learnt today is:
Don’t make a cup of tea until the kettle has boiled …
or
Don’t try and fix a problem until you know what it is…
Sorry Virtualmin
Please accept my apology for jumping to the conclusion that the Update caused my issue.