How much,if any play should there be in the centre support bearing?i have a knocking at between 40-50mph and under heavy load,how do i check the uj's,do i just strip the prop and make sure they move in all directions without too much play,i've been underneath and grabbed the prop and there's a good 1/4 inch of play on the centre support,i would imagine this to be the problem but have been told they should have some play in them so am unsure,thanks for any help.

James, U.J.'s shouldn't really have any discernable play, although you'd tolerate a very small amount. Yes, they should move freely in both directions. The rubber in my centre bearing is sloppy (original at 185k) but with just me in the car it's not really a problem, although I drive gently. Regards.
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BillB

P.S. You may have confused some US readers with "prop"!

there should be some play, though if your able to move it that much by hand its probly time to replace it, it should be tight, its designed to let the drive shaft move freely but support it and also handle the shock of the drive shaft moving around, thats solid rubber inside there.