You might get your speeds back by just changing to any ISP that is NOT Orange, .
With respect, I think that comes from the "pigs might fly" school of thought! I know you think I'm guessing, but it's a simple process of logical thought: the exchange is modern and has good backhaul, and the local wiring in my dwelling has been replaced, but the bit in the middle (local loop) is underground with repair joints at every other house on the estate back at 1km to the cab. Any DLM whether automatic or manual is only going to respond to a problem elsewhere, it's not random.
So it's a wait until the voice phone fails (which from past experience could be a decade or more), as that's Openreach's only legal obligation . I wondered if they might be incentivised to do something in the context of a fibre installation, but the deafening silence in response to that question indicates eloquently, that the answer is "no, of course not"! BT the master in the 1950s time warp of UK infrastructure, and everyone else including the government and so-called regulator (the beggars) can't be choosers.
Just to confirm my prejudices, I've discovered the Visualware tester, which appears to give access to the metrics behind the speedtest, and suffice it that apart from being consistent with TBB auto and other tests and what other posters say about the capacity of my connection, its CoS (consistency of service?) summary comes out at 6% for ADSL (compared to 93% for a fixed wireless alternative I have, even at 5 miles from the transmitter), and the graphing of the results is about as erratic as I can imagine, with a much more block-like graph for the fixed wireless, and TCP delays of 400-800ms for ADSL and 40-100 for fixed wireless. (It suggests the wireless download runs at 12+/12.8 available bandwidth/40 route capacity, and the comparable figures for ADSL 1.1/6.8/24).
So to conclude the thread by answering my own question, I can (and probably should) abandon the phone line and certainly not bet the farm on any improvement!