Yup it would seem I have led you all up the garden path. Ah well. After starting where the cable comes into the house, goes under the floor and then vanishes through a joist, we found, on lifting the fifth random floorboard, a six screw Ashley electrical junction box. A round brown circle. With Four wires into it, one of which on tugging appears to be the main cable into the house.
So I guess this is the star wiring people talk about. Everyone denies knowing it was there. Some happy electrician must have upgraded bt's work something in the 80's or early 90's I would guess. There must be other junctions on the wire but we have ripped up enough of the floor for now. Some wires head off toward areas with no sockets suggesting I am my own bridging tap.
I guess I will have to convince plus.net they should send someone back to rectify the mistake the installer made in the first place. If not the strong temptation is just to try and quietly move the master myself, how hard can it be. But I must of course resist such perfidy.
Anyway sorry for somewhat wasting everyone time, though still I now know what a bridging tap is and a lot more about telephone wiring than I did before.