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Unfortunately even if the street is cabled, sometimes there are legalities that prevent certain properties being serviced. Flats are notorious for this.
In simple terms you have a lease, of only part of the building and grounds. Others can control the common part, structure and grounds. They need to give permission for the cables to be run through those areas before they can reach your flat. For the cable co's that can be too much aggravation to sort out.
To get anywhere you will need to contact ntl:telewest again, and ask them what happened to the site survey.