Hi Mike,
The set up for a 'reserved' IP address is all done in the router. There is no set up to be done in the B2.
The way the system works is that when the Brennan boots up, it sends a request to the router for an IP address. The router responds by sending an IP adress back to the Brennan.
I've just had a look on the web in the TalkTalk forum and it appears that Talktalk has removed the 'reserved IP address' functionality from their routers. There are hundreds (if not thousands) of posts from users complaining about this.
That might have changed recently, so it would be worth checking your router model and seeing if this is the case or not.
Regards,
Peter.