Just phone them and ask nicely.
I had my box swapped out because I wanted an RGB output rather than S-Video. They sent an engineer to swap the box free of charge - the nice man also split my cable to I could get the terrestrial channels as the new box has no pass-thru.
I got the box swapped in Feb '03. I went from a 4001N to a 4010N and the box is great. I've only had to reboot it once in 12 months!
Andy.
With ntl (cabletel) since '97 and no problems!