That is exactly what I was going to suggest you try next, Paul. Put in the tappet, pushrod, rocker, and rocker pivot nut, all as a set so they have run together previously and lapped themselves to fit. I suggest you watch the rocker arm bump during compression to see if it is more convincing than before, and also do a compression test with the tappet correctly adjusted. If results are the same as before, that seems to clear the valve train of involvement. If it clears up the problem, we can talk about how to go further. One option, of course, would be to just leave those parts interchanged, but it would be nice to know what was wrong, for the archives.
Incidentally the exhaust rocker's central pivot does look a bit worn in your picture.
Slightly off topic, the back of the timing gear seems to have been chewed up by the reciprocating oil pump. The pump may have been misaligned laterally and it could also be slamming the camshaft up and down across the slack in its bearing in the crankcase cover. Generally it just seems odd to me, we need to think about it.
