It appears my cam chain tensioner has a bodged plunger. I checked out the parts drawing closely and its certainly not like the one shown. Once I had fitted the new chain from Stewart Engineering, it was apparent that the bodged tensioner no longer worked correctly. The tensioner assembly cannot be screwed into the block, since the bodged plunger is too long and has no smaller spring.
I've asked on the Classic Glory list to see if anyone can get me some dimensions. Just my luck the part is not presently available new, so I'd have to fabricate one. Lucky I have that lathe.
The good news is that the head and head gasket went together flawlessly. The new stud in the back went in easily, and I managed to set the valve timing like a pro, thanks to the Sunbeam Owners Bedside Book.