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Firstly it may well be the case that the property cannot be serviced, even though it appears that cables pass by in the street. Flats can have legal complications in that the cables need to be routed over land or parts of the property that you do not directly control. You only have direct control over your leased flat, not the common parts, common grounds, and probably even the main external walls.
The requirement for you to have a new account is simple. It takes a while for the cable company to recover the costs of installation. Thus they want you to have the service for a minimum period.
As for getting confirmation from ntl:telewest that a cable could or could not be installed, you need to ask sales to get someone from planning to get involved.