@xxgbhxx Just thought I'd do a very quick update.
It happened to me again today and I've finally nailed EXACTLY what the issue was/is and it turns out it was an already known issue with VMWare/PfSense (gee thanks Netgate).
The issue is with VMWares allocation of NIC's. In VMware when you add new nics they number them vmx0 vmx1 vmx2 and so on. When you add a new card for some completely inexplicable reason, VMWare numbers the NEW card vmx0 and then bumps up the interface numbers of all the other cards (so what WAS vmx0 becomes vmx1). This immediately breaks pfSense and pretty much means you have to re-do all your interfaces and firewalls.
SO
The moral here is add as many interfaces from day one as you ever expect to use and if you DO decide to any later on, make sure you fully prep for the impact (because remembering interface names/locations from 9 months ago is not easy!)
Thought I'd leave this here in case anyone has the same issue.