This sounded expensive if this was my problem, so I started with this one. There were signs of a slight weep from the pinion oil seal, so I topped up the level. The diff was filled to the correct level after just a small squeeze of a 500ml bottle, so it took well under a quarter of a pint. I can't afford to ignore a leak in view of the consequences, but clearly low oil in the diff is not the immediate problem.
I also ran the engine and transmission with each side in turn jacked up to make the differential work hard. There were no strange noises.
I had a close look at the drum and backplate, and there was no sign of any polishing (but thanks for the idea Ron). One of the grub screws holding the offside brake drum was only finger tight, but with 4 properly tightened wheel nuts also holding the drum in place, I can't imagine this would be my problem.
The drive shaft flange bolts were all tight (but thanks for the idea Willy).
Having sorted the nearside drive shaft where I thought the noise was coming from, I have had a closer look at the offside one. There is no sign of wear or damage to the wheel bearing. Sight, sound and feel all suggest that it is perfect. The bearing housing retaining nuts are properly tightened too.
The noise only arises on bends, and the things that bends change are the speeds of the inside and outside wheels, the sideways forces on the wheels, the compression on the springs and hence the angle of the drive shaft. If the diff is OK and the wheel bearings are ok, and I know the springs and shocks are OK because I fitted new ones at Easter, then the finger surely points at changes in the angle of the drive shaft, which puts work onto the UJs. Or else give in the suspension arm bushes. Does anyone know the symptoms of these being worn or soft?
Meanwhile, I have focused on the offside drive shaft. I gave it a good shake and thought I could feel a knock, though I couldn't hear one. I am rapidly running out of ideas, so I think I might take it off to look at closely on a bench under a bright light, instead of laying underneath the car with a torch.
Jim