After recently contacting the Plusnet tech support staff a line test was carried out, and based on that the support guy strongly encouraged me to go for an engineer callout; he saw a possible line fault and suggested we could get considerably better connection speeds from our fibre-to-cabinet service. I held off (I'm in the "if it ain't broke" camp) but a subsequent rise in disconnections and a drop in connection speeds pushed me to take his advice. When I requested the callout the staff were a model of efficiency; no time wasting, no need to repeat any information I'd already given.
The OpenReach engineer replaced our 16 years old master socket with the latest model - which produced a modest improvement - but said that given our distance from the cabinet, Plusnet's estimates were somewhat optimistic. Nevertheless he said he'd run checks at the cabinet as well, for completeness.
Clearly he found something to fix because I'm now enjoying hugely improved connection speeds (up from 27 to 39 downstream, 7 to 9 upstream), totally in line with the suggestions from the Plusnet staff.
Bottom line: really excellent support and service from Plusnet, backed up by a diligent and thorough OR engineer.
Moderator's note by Mike (Mav): Post released from Spam Filter.