So, at long last BT sent out an engineer who has actually done something instead of telling me my line was perfect when I knew full well it wasn't. He performed a "quiet test" and agreed that my line was extremely noisy and then used his ladder to check the cable coming into the house which he found to be OK. He then turned his attention to the phone socket and connector coming in from outside and replaced both of them. He also removed some redundant wiring and sockets and all now appears to be well. James from IDNet called me this afternoon to ask how things were. Very impressed!