Some time passes...
The crackles come and go, but I don't use the line much and it isn't impacting the DSL, so I largely ignored it.
Recently it got a lot worse - constant noise. So I logged a call with Plusnet last week, and today an Openreach engineer came around.
First thing I did this morning was pick up the phone, and it was buzzing quite loudly with occasional crackles. An hour later when the engineer arrived, of course the fault had gone away.
But he believed me, and plugged his test equipment into the socket. It showed a slight fault early on, and failed a later test. He wondered if the socket was at fault, and so took it off the wall and tested the pair from the dropwire.
This passed the tests, so he fitted a new socket.
So far, it's all working fine.
Curiously, annoyingly... if you read the start of this thread, you'll see that the crackling has been there for some months, and early on during diagnostics, I'd damaged the socket and so had replaced it. Therefore the original socket was showing a fault, and the one I'd replaced it with showed the same fault.
I am thus hoping that I'd replicated the original fault in my repair - the alternative is that the intermittent fault has just gone quiet for a bit and will come back in due course.
Also disappointingly, the DSL stats are pretty much the same as before - I was imagining a line fault under the road somewhere that would miraculously give me extra bandwidth when repaired
Still, if it's all fixed, I'm happy
Ian