Not much more to this thread, just a footnote.
My connection is now back to something reasonable using my favourite 2WIRE ADSL modem/router. Currently 9853 kbps @ 5.9 dB.
One of Plusnet's recent notes states the following.
(LL14 Task) - Engineer’s notes on 17/08/2011: Line Test not carried out. Test facility was not available. REIN issue identified on line, had co op call with REIN helpdesk and was told that a REIN fault could not be raised unless the line had an AC bal of 50dB or above and an RF3 filter was fitted on the line. The AC bal is above 50dB and engineer has fitted an RF3 filter and advised customer to monitor the broadband.
I have a question about the RF3 filter. It is connected upstream of the master socket so Openreach's property. Is leaving this in place a good thing? It was expected to be a temporary requirement so it is not fixed to the wall. It also has a short lead going to an adjacent earth point. If it is worth keeping I will just fix it to the wall properly myself. Otherwise I may ask my ISP to arrange for Openreach to remove it.