Thank you very much for your reply.
I know the router we have will support more than one X box playing with open Nats because the technician we spoke to at Virgin Media confirmed this was possible.
It is not knowing enough about how these things work that is causing the issue, because I have no idea how to facilitate it.
I have been able to access the routers home page and I have checked that the 'plug and play' is enabled...I just don't know what to do now.