So I took it to bits yesterday. The bolts were bloody tight, as you might expect, so careful cleaning was essential. The doughnut is knackered; I can twist it through ninety degrees with my bare hands, as though I was tearing a real doughnut in half.
It looked fine but on the prop' there is no real way to test it, so it goes to show how knackered these things can get whilst still looking OK. The yoke-shaft (just made that up - I don't know the correct term 
) slid out nicely, with only the tiniest amount of play detectable, but on examination things were not right. The two bearing surfaces on the shaft which run in phosphor bronze bushes pressed into the propshaft looked nice and shiny but the wear patterns are not identical; the bearing surface on the end of the shaft (furthest from the yoke) shows a clear zig-zag wear pattern caused by the doughnut flexing and shortening as power is fed on and off, the other bearing surface showed no zig-zag pattern and when I examined the PB bush it just pulled straight out of the prop! The inner surface of the bush looks just like big-end/mains shells which have suffered acid corrosion with 'slug creep marks' very visible. Funnily enough the O-ring in the end of the prop' was intact and there was no sign of water ingress. The bush slips onto the shaft surface easily with very little play, so I wonder from the wear pattern if the bush and shaft were rotating.So it's a new doughnut from Baxter, and the shaft off to a Mate's to tidy up the bearing surfaces on a lathe before fitting two new oversize PB bushes. Re-assemble with two new UJs (yet again!)then a dynamic balance before refitting. If the bloody thing vibrates after that I'm buying a 2WD.
Incidentally, the flange on the end of the prop' that carries the whole doughnut assembly is pretty naff engineering-wise on my prop'. The flange is so off-centre that it's possible to see daylight if you look down one of the tappings that secure the doughnut to the prop'!
This probably explains the massive balancing weight that was welded on at manufacture.