You got your connection sorted then BE What was the cause?
In the end, it came down to a faulty faceplate filter that was causing disconnections simply by using the phone & a dodgy joint between the underground cable & the pole top DP across the road from my house that caused random disconnections, increasing in frequency as the weather became warmer & dryer.
Some of the other engineer/technician visits where "work" was done caused slight improvements (usually simply by re-making poor cable joints & onceover replacing/rerouting the drop wire from the pole).
I believe the dodgy joint had been the main issue all along & the faceplate issue being more recent as a number of faceplates had been replaced previously, with no real improvement seen.
We had a power cut yesterday morning that resulted in a slight increase in DS & US sync & throughput speeds:-
From experimenting with different things over the problematic months, I/Plusnet discovered that my DS sync speed increases by around 5Mb - stabilising at approx 33.5Mb when US is capped at 2Mb, rather than what has now become Plusnet's default "up to" 10Mb.
This also appears to be the position for other sub-40Mb users on being switched to the 10Mb US service.
Users capped at 40Mb but with higher attainable rates still achieve 40Mb sync speed with the benefit of increased US sync speed.
Even before 10Mb US became the default Plusnet offering, I chose to have the increased US throughput speed at around 4Mb instead of 1.67Mb, at the expense of some DS throughput speed due to the nature of how I work from home fairly often.
My gripes over this matter are about how long it took to actually find & fix the faults & the constant "your connection is working perfectly satisfactorily & within our supplier's" guidelines" type of comments, despite me providing graphical & raw data evidence to the contrary.
Even reporting a number of times that engineers simply climbing the pole to work on other users' lines caused disconnections, thus highlighting a probable loose joint issue took over 3 months to be even looked at & that was only because I begged the visiting engineer to check it out. It hadn't been mentioned to him by BT (or Plusnet's notes).
In the end, that turned out to be the main culprit.
So, all in all, it was a very astute decision on BT's behalf to supply locked modems where probably the vast majority of users have absolutely no idea how poorly their connections may be performing at any time & thus fault reports are kept to a minimum & it is incredibly difficult to dispute BT's claims that their aged infrastructure is "fit for service".