Reader, it was the universal joint.
It was quite a relief, because
- It's always much nicer to know what a problem is, than to have a problem and not know what it is
- It was something I could fix myself
In obedience to the instructions, I applied gentle blows to the yoke with a copper mallet.
And then rather less gentle blows.
and then I used the blowtorch.
And soaked it in release oil.
And hit it lots.
And put it on a piece of teak (rescued from a skip some time ago. I knew it would be useful one of these days) and whacked it even more.
And finally it came off.
And then all I had to do was cycle across to the south side of Bristol for a new UJ (as we mechanical types call them)
And put it all back together again.
And now the car is working beautifully. At last.