Kitz, thanks for that helpful reply, which helped converge nicely on the solution. I think I'd done most tests, and was about to (re)try the SNR at master socket with phone off-hook, when the god of poor communications intervened. There at the inspection point outside the house next door was a BT guy, inspecting a bundle of underground wires. I asked if he could check my own, but he was just doing an installation, and told me to report a fault. At exactly the same time, my line dropped again irremediably, and the line crackle increased tenfold, which made diagnosis much easier. Even their 151 auto-test system picked it up. Yesterday, another BT man repaired a bad joint on my line at the same point, and my SNR margin went up to a nice 21, at least 10 higher than what had been the mode previously.
I had in fact been noticing slight intermittent crackle, along with the BB deterioration, some weeks ago, and asked BT to do a line test. The person who rang me back said the line was ok - presumably he was concerned just with voice quaility - but did seem to suggest that I might get back to them if satisfied that the problem wasn't an internal one. Which is where I came in with my first posting.
What I've read about BT's charges if they're not at fault was a bit intimidating. so this week's drastic deterioration was very reassuring. If it hadn't occurred, and I'd simply passed on my findings to my ISP, I wonder what the outcome would have been, and whether it would have been resolved so quickly; and whether even BT would have recognised and rectified what would have been essentially just an intermittent BB problem.