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You say you have been "given" the DI4000-n. That suggests to me it has not come legally from telewest themselves.
Simply put you subscribe to Telewest as standard and rent from the one STB only. If you want two STBs, then you pay for the exta box as part of your subscription.
The smart card is coded to particular STBs. If the coding needs changing that has to be done by Telewest. If your current STB is inadequate in some way then it's for Telewest to agree is needs replacing and send an engineer to do the job.
The STB you have been given is in fact the property of Telewest and should not have been given to you, but returned to Telewest by the original subscriber.