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The simple anser is yes this can be done, but it may take a bit of persuading ntl that you really need it. Obviously there is a cost to them as an engineer needs to come and fiddle about with the wires at your house.
The other issue is that at present even the Samsung STBs are limited in their maximum broadband compared to the modem. A a result many are clamouring to get changed to modems so they can get the higher speed, placing demand on ntl. You've just got to see what they say when you call them.