2) ....apart from "the worst ADSL ISP", BT are lightyears ahead of TalkTalk/Tiscali.
How very, very true. I was with Nildram for Broadband since 2002. Slightly dearer than others at the time but top in all the customer surveys, etc. that I read. Rock solid and on the odd occasion I had a problem easy to reach tech. support and prompt action. Fast forward to various takeovers culminating in my contract ending up with Opal (Talk, Talk).
A disaster of epic proportions from day 1 and truly impossible to even get through to tech. support in under 45 minutes. When my up to 8Mbps dropped to a consistent 0.2 Mbps and after 2 months of getting nowhere I turned to BT Infinity. Installed on the day they said and been brilliant ever since (10 months).
From reading the OP's problem it is highly likely that the long distance wiring from exchange to house is a likely suspect. Very frustrating.