Thanks Dave, in my frustration I've used many different ways to check the valve clearance. The consensus on the 'net seems to be "tight is best" and that's sort of always how I've set them.
Just thinking out loud, but although I've changed the cam bearings in the past, I wonder if they have worn a bit again? My thinking is that I set the clearance at TDC where the cam is unloaded and thus settled as far away from the rockers as it can be, but the crankshaft will try to push it upwards towards the rockers under load. The camshaft may also be rocking a bit along its axis if one side is driving a valve. I'm just wondering if there's enough slop in there to take some clearance away when it's running??
I've also never checked the clearance at anywhere other than TDC. I'll check the clearances at different points on their respective base circles next time the valve cover is off.