Thank you very much for the replies, it' s great to see an active, helpful forum - fills me with much more confidence :o)
Using these tips I have now successfully created some rules for OpenVPN, mail servers, HTTP ports etc. It' s looking very ' ZyXEL-like' now!
I had consulted the online docs, but the examples I' d come across seemed to go into great detail for creating advanced VPNs or using DMZ' s etc. For my first step of creating a straight-forward port opening/forwarding, the docs left me a little intimidated and confused!
Thanks to you guys though, I am making great progress