Spend Friday night pulling the head off the engine.
Actually quite lucky... Headgasket was blown between cyl no.4 and a water gallery. Pistons all look good with no markings.. Thank god for forgies 
Many thanks to YelloRolla for lending a headgasket and a hand at dismantling/reasembling!
Engine went back together Saturday afternoon. Still some finishing off to do but almost there.
We fixed the crank trigger disc so there is no way it should slip again and throw the timing out.
Here are a few pics, some even slightly incriminating 
Here you can see the blown headgasket.
Undamaged block.
Repairing the blown headgasket.
Whats this tool for??
This seedy looking character is responsible for the photos and also came up with the suggestion to use hotglue on the damaged gasket.
Torquing head back down with new gasket.