you can also isolate the problem by using different runs of known-good patch cord

Care to elaborate? As you might have guessed, I've been down the pub with Hugo, so I'm not visualising this as clearly as I might do.

Otherwise, thanks to everyone for the plethora of advice. I'll investigate some of these avenues tomorrow, when I'm sober enough to work a wire stripper without endangering life and limb .

Of course, each failed attempt means that I have less and less wire at each end to play with, and I'm getting closer and closer to the wall. Pretty soon, I'll need to run a fresh stretch .