I just tried shining a torch on the side of the shifter opposite the cable's entry point, and found that the cable doesn't line up with the hole for the short (approx. 1cm?) tube that sticks out of the right-hand side of this left-hand shifter, and guides the cable. I don't know why this one's misaligned, but it is. Tomorrow, when I've recovered my patience

, I'll try guiding the cable at this spot, so it gets through that final hole. I don't want to remove the shifter if I can avoid it, because then I'd have to remove the brake cable also.
The shifters are the original ones that came with this bike, which has now done over 72,000km, so perhaps I need to thoroughly clean and grease them, or even replace them? They work really well, though (once the bloody cables are in them, that is!).