I finally fixed my NAT type!
I got myself a NETGEAR FS605 10/100M Switch, so I don't have to compromise and have wireless on either my PC or my Xbox360s whilst one of them uses my ethernet cable (didn't want to get another extra long cable, couldn't be bothered with the hassle!)
I purchased the Switch and two 1m long ethernet cables for both my PC and Xbox, got them both working ok at the same time, well I say 'ok', they work fantastically, as it feels like I have 50Mb broadband on both now, which is what I'm paying for!
I unplugged my router from the wall, and plugged it back in; made a fresh start on my router Control Panel by deleting all of the port forwards and everything. Left it in uPnP and it just worked, without any other amendments!
Conclusion: Happy Jay =3