hmm broadband is annoying with tv. All i can say for definite is, try to avoid long stretches of cable as this will reduce the quality of the video.
You can buy higher quality cable called ''double-shielded'' coaxial cable,(dont know if you already have it, Sky give LQ cable for indoor, but i dont know about NTL) it's for out-door use but can be good for indoor too to increase quality etc.., sorry i dont know where to get it but i would take a wild guess that you'd get it in Maplin, but they can be quite pricy.