So, several months after this problem started, I've finally got a solution and some clues as to what happened.
Eventually plusnet sort of gave up. They half heartedly offered to send more Openreach engineers but with the caveat that with so many engineers already having had a look at the line, they weren't sure what use any more engineers would be.
So I decided to take a risk and switched to Direct Save Telecom in the hope that the problem was due to the backhaul and a new provider would mean a new backhaul.
Initially the problems persisted (interestingly the plusnet connection still worked at least until the following day too - I was expecting it to stop working at some point on the switchover date). I spoke to Direct Save's technical support a couple of days after the switch and they said that they would investigate and get back to me.
They never did get back to me, but a few days later things mysteriously went back to normal. A couple of months later I phoned them up and they said that they had changed some kind of traffic management/priority setting on the line with their suppliers and that seems to have done the trick!
Edited by ec5779 (Tue 17-Sep-19 01:49:53)