[Monkfish]

Well, I can't get in contact with anyone at NTL to get an engineer sent out. High tech problems aren't catered for by telephone call services and there's no one with any technical knowledge to understand the problem and get an engineer sent.

Anyway, I used some clear tape on the thread to isolate the screen and that's done the trick. It also hasn't affected my connection quality. This is probably because very few people are tapped out from our street cabinet. My signal is so strong I didn't even need to remove the 12db of attenuation the engineer connected in-line. I was worried about my up-line bandwidth however, but that too seems fine.

As I suspected, the frequency of the broadband signal is so high that attenuating the lower frequencies made little difference. It did, however, remove the interference on my speakers, which would suggest the source impedance of this interference was in fact quite high (or maybe just the 50Hz component was audible and that's all I needed to block).

I can understand why it might be law for the screen to be isolated. Earth-loop/mismatch problems can be potentially dangerous. I wonder if isolation is an area where NTL is skimping?
I might have another attempt phoning NTL and simply ask for an engineer to be sent out to fit an isolator (under the KISS principle ).

No, I haven't got the TV service, but I will remember about those earth-loop audio isolators from Maplins. Thanks.