A little update...

With a lot of persuasion, PlusNet got a BT engineer to pay us a visit. Despite my stressing to the engineer that at times the connection is so unreliable, he concluded that it was within specification and nothing could be done.

PlusNet agreed to lower the BRAS profile to 500, which has improved the situation slightly, yielding a downstream noise margin of about 15dB.

Is there anything else we can do? I never did try taking the ring wire out as I don't fancy messing with someone else's wiring!

In reply to:

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Is there anything else we can do? I never did try taking the ring wire out as I don't fancy messing with someone else's wiring!

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So you elect not to do the thing most likely to be of benefit and then ask for other ideas ?

Precisely. If you can't help yourself, you're going to have to live with it.

Fair point.