Thanks for that - would seem that the key is in telling eth0 that's it is EXTERNAL as that's the only thing that I have had to change and it now works on a static IP.
I had left this alone as, on the initial "stand alone" installation, eth0 had been from DHCP and had "found" the correct gateway, but even although it was showing in the dashboard as "internal", clearly had full internet access.
I was further confused by the box being marked as "WAN" as I had presumed that this applied to a twin NIC interface where one of them was dedicated to WAN and the other to LAN.
Anyway - it now works the way I need it to, so, assuming that I have not broken anything else, I can continue with exploring the server and possibly expand from just using it as a NAS.
If anyone from Zentyal development ever reads these threads, they might like to include this step in the documentation relating to network setup.Oh Dear
- spoke too soon. A re-boot of the system has left me without internet access again. Had to go back to DHCP provided IP on eth0 "up, internal, link ok" and auto-gateway "dhcp-gw-eth0"