Finally an update!!
The passenger side (odd numbers) cylinder head is in my living room, as is the remains of the rocket shaft assembly. The rest is some where in old oil!
There was evidence of a blow through the gasket to the outside, explains the leaky exhaust i couldn't find.
A couple of the rockers were badly worn, but all past their best. That's the source of the knocking I've been getting!! If course i will only know for sure once it's running again with a new rocker shaft.
The related lifter also was starting to show wear on the cam face.
Also wear in a couple of the lifter bores, but i don't think there is much you can practically do about that without huge expense.
Pics to follow shortly