Hi Colin,
I have a Mk3 Pamp with the endfloat problem although it doesn't have any issues with vibration. The problem seems to be the interference fit of the main bearing on the crankshaft is not quite right and I gather this is quite common. If you grasp the flywheel you can move it about 1mm (endfloat) which isn't ideal but i'm not pulling it down until it needs mains and seals as it runs fine. I have heard stories of hard chrome plating on the crank to get the interference fit correct although this sounds quite involved and possibly expensive. A new crank will set you back about 320 quid tho.
Tony.