Thanks, I thought that might be the problem. I was hoping that the port forwarding would make the internal ip connection behave in the same way as an external one.
My ISP is Sky, and they lock their modems down very tightly (which is the main reason I use the extra router, so that I can use OpenDNS). I doubt if I will be able to get it into a true bridge mode.
Anyway, thanks for the help.