Update 04/04/2012:My own engineer's visit is now confirmed for Saturday am.
Email just received from Alex at Plusnet:-
"BT have confirmed the appointment for 07/04/12 AM.
I will add notes for checks for High Resistance and if possible the completion of a TDR or FDR test."
Hopefully, these notes will actually be passed on to the visiting engineer.
Not knowing for sure what a FDR test was, I looked it up:-
"Frequency Domain Reflectometers (FDR) test cable at specific frequencies or frequency bands.
Time Domain Reflectometers (TDR) test a cable against the manufacturer’s Characteristic Impedance, which basically tests the cable for any and all frequencies."
If there is any issue with the engineer being uncertain which specific frequencies to test for the FDR test, I'll whip out my Hlog graphs as a last resort.
However, after all these months of intermittent instability I now basically want/need the whole lot testing (& RE-TESTING AFTER ANY WORK IS CARRIED OUT).
Who knows, 40 Mb may just be on the horizon