I have been down the poor reception road many times with NTL-not my favourite organization-I bought a booster several years ago and I found it only introduced other problems -it can make the signal too strong on some channels-which is as bad as a weak signal so overall it was a waste of money.
There have been times in the past when I complained to NTL and their Tech guy is able to "tweak" the signal on the box on your house-though this does not last in my experience.
It might be worth considering their digital service -while not faultless delivers a much better signal.