Many thanks to all for the excellent and detailed advice, especially to
Mr Sheep:
Our 'Bible' (ISIS Library) states that ....... 'It has been found that the SSFP RF3 filtering functionality has some limitations and will not prevent REIN faults in all cases.
The existing stand-alone RF3 may still be required to assist in solving Rein Faults, and can be fitted even when the Mk2 VDSL SSFP is in situ'.
Things were more or less as I had guessed — in fact, I had added a replacement RF3 to my parts order, so I could try clipping out the choke if necessary, but I suspect the line will be better with it in as all the flats round here I've managed to check have an RF2 from before the days of ADSL. (There's a mast on the hill behind the house relaying, as far as I know, AM, FM, and TV for most of a town of 50,000.)
I'll let you know what happens if I find time to try the experiment, but my goals are more modest than most of you here I think: right now I'm just trying to turn a slow and unreliable connection into a solid and reasonably fast one.
Besides, it's sometimes not the wiring technology,
per se, that makes the greatest difference. The biggest improvement I ever made to an ADSL installation was to buy a new PSU for my downstairs neighbour, to replace the piece of junk he was using with his laptop (and thereby enabling the whole block to receive AM radio, into the bargain).!
As a friend of my replied to his ex- when she asked him what he thought of her fabulously expensive new hi-fi: "Well, I think you could have gotten the same improvement cheaper if you'd just switched off the washing machine and shot the dog!"