Cracked Camshaft Journal

Having now disassembled the Cylinder Head, it's now clear what the problem is. (Cylinder 1 port was a furphy, it was the beginings of a wasp nest.) A broken camshaft journal has allowed the intake camshaft to jump out of the "C-ring" and move around in the head.

The noise was actually the camshaft lobes slapping the head

Extensive damage all the way down the head

Minor scoring of the cam lobe from the metal filings off the head. It can probably be cleaned up/repolished if need be. As the camshaft is made of a harder metal than the head it is pretty much undamaged.

 Next step is to find a new cylinder head, preferably complete with valves/shims/buckets/cams.

Cheers,
SammyA