I had a letter right about time it was in for MOT.
It needed a new o/s/f wishbone anyway, as the ball joint had play in it, apparently, so I had it replaced with a used one from a Californian 4200.
No cracks were evident in the US arm, however the bushes are still in my old arm, so I can't tell. It looks fine externally.
It has to be said that the ball joint in my arm seems fine, with it off the car, so maybe there was play with weight on it...
So, I have a spare wishbone if anyone needs one to investigate this issue further or use one for prototyping anything.
Antony