No matter what NTL has done, whether you have had the service installed or not, if a salesman came uninvited to your door and you signed something there and then, you are legally entitled to take advantage of your 'cooling off' period, which is seven days. Admittedly it is highly uinlikely that they will get your service installed within 7 days of you ordering it, but if they did, you still have the right to cancel your 12-month contract with NTL. However, you might find that you remain liable for the cost of the actual install itself.
Useful info and FAQs from the Consumer Protection Regulations 1987 and 1998 are here:
http://www.dti.gov.uk/ccp/topics1/facts/doorstep.htm