Interesting development. I increased ram to 4gb and updated FreeNAS to 11.3-U4. On reboot same problem. VM went down after about 12 hours. Subsequently I happened to be working on a Windows machine - I think this is incidental, but I want to explain just in case - and had a Putty terminal open into the VM while I worked for about 8-12 hours or so on Windows. The VM remained connected and I then decided to leave the Putty connection running overnight. Following morning, still connected, so as I was going away for 4 days I opted to leave the Windows machine on and Putty linked to the VM as an experiment. During this time checked the VPN connection to the VM outside the LAN and it worked every time. On return about 3/4 days ago I exited the Putty session and switched off the Windows machine and the connection has persevered ever since. Total uptime is now 8 days.
Note, however, that on return about 4 days ago I also deleted a couple of other VMs that though they weren't runnnng contemporaneously may have contributed to warnings I had been receiving about over commiting resources (now gone) when I launched the ubuntu server after each downtime. I have also since deleted all the snapshots associated with FreeNAS for the main pool and then recreated one main one.
One other thing I notice is that the 'users' command via a Linux terminal into the VM shows 2 users of the same name - the only other user besides root (don't think I created a root user when the VM was built). Since I am not logged in via the GUI shell or any other device, I cannot figure out why the same user is logged twice. In any case, could this second login be the reason the VM remains connected by keeping it alive somehow? In relation to Patrick's comment above about Virti0 I am still using Intel. With regard to Samuel-Emry's advice on connecting via a serial session, I tried this but obtained some kind of error. Unfortunately I didn't log it so cannot now remember what it was. Apologies and thanks for the help. Will update in due course.