A bit of reading around the subject matter offers a good deal of insight. This is not a conspiratorial "they are out to get you" space, but true space governed by the laws of physics.
"but as the cabling from the exchange to the cabinet is fibre, that wont matter, will it?"
Such thinking ignores the consideration that the connection from the cabinet is copper, the signal is far higher frequency than ADSL and therefore the signal fall off with distance is much more acute. Here is some helpful back ground reading - VDSL2 (FTTC) Speed vs. Distance | Increase Broadband Speed - unfortunately the page does not seem to be delivering the very informative graph, but the text is plain enough to read...
"Speeds fall to about 60 Mbps when 500 m away from the street cabinet and, at 1 km, about 28 Mbps connection speed can be achieved."
So if you are actually 1km from the cabinet, then your experience is somewhat above expectations. See this graph for an illustration of speed fall off on distance from the cabinet - Chart-FTTC-speed-against-distance | Increase Broadband Speed
You will observe that the speed fall off gradient is the steepest between 400m and 900m which covers your probable distance. Therefore you can expect that any marginal variation in line and environmental characteristics might result in marked changes in performance.
As advised before, signal power levels are adjusted so as to achieve optimal speeds whilst not interfering adversely with the speed enjoyed by others. The speeds reported at contract renewal are those provided by BT Openreach (not Plusnet) and will reflect the actual observed average speeds in your location. Are BT OPENREACH lowering the minimum guaranteed speeds so that they can deliver a better overall service to users across the piece, would be an interesting question to raise with them. One thing for sure though, this is NOT something being done by Plusnet.
DLM history - that was a question asked of the staff members I tagged.
"general pain in the backside issues which shouldn't keep repeating and shouldn't, seemingly, be not able to be found and definitely not able to be repaired"
Do not think I have seen such reported here - you've inferred that you feel you are getting less than you think you should year on year, but you have not explained how a little less speed actually impacts your use of the internet. Unless you have a houseful of people hammering the internet a few mbps here or there is unlikely to have a material impact on using the internet. Sub 10mbps is more than adequate for watch on demand viewing. If that exhibits buffering then there is probably an issue elsewhere.
Copper is a vulnerable conduit, if can have been fixed yesterday and be broken today. Unless an engineer checks every joint, there is the possibility that the most obvious one DETECTED that day gets fixed and another marginal one shows no issue THAT DAY. I have experienced the local circuit breaking 30 minutes after being repaired - sometimes copper wires (certainly aluminium) will stress fracture where they have been worked on.
Unless the staff can identify the DLM disconnection reasons and no fault is found on the circuit, then if you NEED better than 34mbps you will either need to move closer to the cabinet or switch to FTTP, which delivers the same speeds everywhere, irrespective of distance. Well effectively it does, light only travels at the speed of light so travelling twice the distance will take twice the time, but I doubt not even the most discerning will notice the difference.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
