360 NAT Issue

[#1]

Package: Gamers
Router: PlusNet Supplied DSL-2640B
Connection: Wireless, using official adapter
Config: UPnP, no port forwarding.
My Nat is currently set at Moderate. I can make this open by adding the 360's static IP to the DMZ. However I shouldn't have to do this, what's the best way to get around this?
Looking on the support pages it only tells me how to configure the Xbox to talk to the router.

[#2]

Don't think you need a DMZ. I don't and wouldn't.
Assume all your XboxLive connection tests are OK apart from XBOXlive itself and NAT.
If so, try opening up Xbox ports on your firewall. Check out at
http://support.microsoft.com/kb/908874/en-gb?sd=xbox
Just done it myself an it worked
AnD

[#3]

Tried that but it came up with moderate NAT rather than open. Still leaving it in the DMZ works however.
Still at a loss as to why it's moderate...