Thanks to everyone who has contributed to this thread. I am now utterly convinced that the cable is the culprit, due to earlier symptoms that I observed but didn't pursue: key working (effectively) in one direction only. I can't recall, however, if it was opening or closing, but it was only two or three times before the failure.
To dot the Is and cross the Ts on this topic, all the components for making cables are available from
https://www.venhill.co.uk They will also build from a diagram or a pattern. I like the allen screw-equipped nipples. Very neat.
Derek.