Thanks a lot for the reply, mate.
Well, I'm sure the firewall ports are open because I even experimented with opening all ports... Nothing.
I guess the answer lies with your reply about the dedicated server. I've been running it on my PC thinking that all and sundry could play on it regardless of platform...
I find it really bizarre that multiple platforms can't play against each other... I mean, it's the same gameplay surely? So long as everyone's playing the same game map what's the problem? A weird omission IMHO.