Tom,
Many thanks for your help and the very useful link. I got the lens out, flooded the mechanism with lighter fluid and generally tinkered around but ... still nothing doing. I was given this camera, so abandoning attempt at repair at this point feels right to me. If it was a family heirloom or better than my Leicas, I might have invested a few more hours in it, but I'm going to cut my (time) losses now and abandon the project. If there are any bits you would like, I'll mail them to you for free.
Now ... the spy story.
The camera was - as I said - given to me by a generous colleague in working order. (It was my setting the self-timer that jiggered it). My colleague was so generous that when a young Czech woman frequently asked to borrow it in the late 60's/ 70's - I forget which - he gladly lent it to her. What he didn't know was that she was using it to photograph military aircraft and installations and pass state secrets - obtained from her Air-force boyfriend - behind the iron curtain. She was having the films developed under my friend's name until a suspicious d&p employee contacted the police who involved the secret services who in turn came knocking at my helpful colleague's door. As far as I know, the cold-war spy managed to flee the country before her arrest. My colleague is still on a database of potential security risks, as he found when he applied for a job teaching in a prison.
Best wishes,
Seán.