Stripping your connection back to the bare line with only the ADSL modem connected is a good confidence check. If you know you are right and it is a Corus cable problem, you can push them with confidence.
I have resorted to exactly that in the past. Fault turned out to be a bad pair on the main cable in my street. I'm sure that bad pair got passed from house to house because every 9 months it would be the same problem again.
As people in the neighbourhood migrate to fibre, the copper should get less noisy.
Edit: Spelling.

