Nope, it won't work AFAIK.
I beleive NTL and Telewest require all their digiboxes to have viewing cards in them, and for them to be registered with the servers and linked to an account (otherwise they will not normally work).
Also, all of the boxes are technically NTL/Telewest's property and as such you should not be able to buy them.
About the only thing you can possibly do, is use the connection straight to your TV/VCR to get a good signal for channels 1-5 (analogue), as I beleive they are sent unencrypted - but that could cause problems for your digital box, or other users in your immediate area (and if they trace it back to you, you would get a bill for sorting it out).
NTL and Telewest both other second STB's with viewing cards that give them mirror subs fairly cheaply I beleive (something like £12 a month), which includes the rental and installation of them.