Adam sounds like you have a line fault there mate. Or your connected to a faulty port at exchange.
In any thing like that go to your master BT socket and disconnect anything extended on that. If you still have any issue then it has to be a BT/NTL/Telewest fault.

Had similar at mine. It turned out bt had used scotchlocks on the join from the cable from the outside to my internal cabling. when i pulled it, it fell off in my hands. They had a fault raised for 2 months

If your cable/line is shorting to earth or indeed a voltage this will knock out your ADSL.