Recommendations for routers:
http://www.cableforum.co.uk/board/article.php?a=74
If using wireless you need to think about security too
http://www.cableforum.co.uk/board/article.php?a=12
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Remember the fundamental, when you do add the router, power off the STB / router / PC unplug old cabling and setup the new. Only once cables are in place, repower the STB, let that settle, then power the router, let that settle, then power the computer.
Using a STB it's easier to set up a router first if you use ethernet between the router and PC. Later you can then add the wireless bit. You will need to register the router with the ntl STB. First time you try to browse the internet you should get to a ntl registration screen, if not enter start:ntl in Internet Explorer's address bar. Dig out your broadband customer registration details.