I've got a bt homehub router, here's what you need to do:
enter bthomehub in your web browser
go to admin, enter your username and password, usually admin, admin
go to devices and select "edit" on the xbox device in the list
in the applications drop down list at the very bottom, select Xbox Live, then add it to the applications list.
Click Apply
Make sure you xbox isn't on
Select the restart option to soft reset your router (not the reset factory settings one!)
Your xbox should now be NAT Open!