Thank you everyone for your suggestions and for your patience with my ignorance.
Please, before I start irrevocably cutting wires that don't belong to me, indulge me with one more question. Right now, the brown 'bell' wire in my diagram is effectively tied to one of the signal wires via the 1.8uF capacitor in the redundant master socket. When I disconnected it at that end, my upstream rate fell sharply. I *guessed* that the disconnected 'floating' wire was somehow picking up RFI and then feeding it into the signal wires.
So: if I remove that socket and all its wiring, should the brown (and green) wires be connected inside the new master socket? If so, to what? If not, will the same phenomenon occur?