" There must be something unusual about this case for it to be just running on without a whole heap of things already being ruled out."
rotting cables (140 metre run in my case, which has dropped to sub 16MB/s at times)
openreach playing musical cable pairs(*) when people in the street complain (at one point adding 40m to the loop (August 2020) - which was removed on Friday when the circuit was transferred to a "new cable". Some poor sod got bumped to make that happen and won't be happy...)
(*) "Find the least bad pair, move the complaining customer to it, too bad for whoever was on it" - and yes the Openreach contractors freely admit to doing it on orders of Openreach managament. They also freely admit the in-ground cabling is rotten but claim that Openreach management refuse to deal with it by replacing them and force them to simply keep bodging. (I have to wonder who got dumped on my old bad pair and how long it'll take before I get bumped again)
Pits flilling up with water every time it rains (resulting in DSL line problems starting about 24 hours after sustained rain does and clearing up a few days after is stops)
Apparent overload of the processors in the ECI cabinets - lots of PPPoE dropouts and multiple customers complaining over multiple ISPs - Openreach flat out lied to Zen about this, saying nobody else was complaining, only to have a tech show up mentioning he bumped into another tech at the cabinet working on an identical fault for a different customer/ISP
Apparent overload of the backhaul to the exchange head end and/or the head end crapping out regularly (the techs have all mentioned this and the number of callouts they get)
These problems have persisted over 5 ISPs, 15 years, 5 routers, 3 NTE5 blocks, 5 faceplates, 2 drop cable replacements (and there´s nothing on the inside end of the line other than the router, no extension cabling at all and indoor cabling runs a whole 4 inches)
And yes Zen DID say "Nothing more we can do"
The distribution and trunking cables in this area are lead sheathed paper insulated - and the ground is a swamp (quite literally, itś in the Doomsday book) with very high water table.
Couple that with 40 years of the local council not clearing drains (surface flooding has been a constant issue) and it seems the pits have been soaking in dirty water for decades and any breaks don't have sufficient air pressurisation to keep water out.
That there's a water issue is underscored by my Fritzbox reporting "Bridge taps" of varying lengths and varying distances along the line when the weather's been bad for a few weeks. Real bridge taps don't move around, this is insulation breakdown/foreign battery raising its head
Openreach have been actively and openly hostile to getting this fixed and I'm far from the only affected customer on the cabinet (at least a dozen people in the vicinity chave confirmed similar problems with both DSL impairment and massive levels of PPPoE dropouts)
It's also far from the only area suffering similar problems. The problem is OFCOM and a large part of the issue is "Regulatory capture"
Ofcom refuse to take individual complaints so Openreach can get away with shafting customers (one of very few regulators worldwide who refuse individual complaints) for years, as has been repeatedly demonstrated.
"We don't care, we don't have to, We OWN the regulator"