have you done this ONLY on the backplate

I have now. It gets strange at this point. The backplate is as you'd expect -- the only 1's in my truth table are down the diagonal -- i.e. 1 goes to 1, 2 goes to 2, but nothing else.

Now, as soon as I connect the cable to the other side of the block, I get the same behaviour as described before: blue, blue/white and green become common.

Right, further investigation reveals that I'll have to hand back my "king-of-crimps" crown -- my "known-good" cable ain't so good. Somehow the (you've guessed it) blue, blue/white and green cores are common .

Beginning again with another piece of cable -- this time one that I've checked properly -- not just for continuity but for inter-core isolation, we find:

That the truth table is now correct.

And, in fact, after faffing around a bit (i.e. crimping it a bit harder) with my (now checked properly cable), that I've now got connectivity with pin 1.

Hmmm.

I'm going to check one other piece of cable, and if that's good, I'll put it all back together and let you know how I got on.

It's a little embarrassing, and I'm sorry to have led you all on a wild goose chase, but at least I've learnt something....