It's strange that the WEP key isn't listed on the barcode sticker on the bottom. You can access the router GUI on
http://192.168.1.254, go to Local Network then Wireless Settings, set it to use a custom encryption key and enter 10 numbers of your choice as the wep key. BT don't have a copy of any of the WEP keys assigned as default to the routers, if you call them they'll probably just take you through what I mentioned.