I swapped the worn joint for less worn one I scrounged from a friend. This
has almost got rid of the strange wobble/judder. It still didn't feel
perfect when I took the bus on a quick test-drive, although it could 
just be my pessimistic imagination. I will try swapping the shaft as well
in case it is bent, as someone suggested.
I stripped the old joint. It had not been making any noise, had plenty of 
grease and had no play (backlash) when checked by hand, but a some of the 
balls were badly pitted and it was definitely knackered. Looks like I'll
have to buy a new joint after all :-(

Andy E.