James, thanks for that
Good, interesting information!
One reason I was reluctant to apply any lube of any sort was that the old inner was very sticky (with oil or grease perhaps) and in consequence had picked up a load of contamination on its exposed parts and this, I felt, may well have contributed to its failure.
On my bike there's very little conventional outer at all, just a few inches from the shifter to the frame egress point and then another few inches from the frame exit point to the deraillier. I did blow these two short lengths of outer clean with the air line.
The gearchange is now beautifully silky-smooth and precise; a joy to use in fact and better than it's ever been