So the engine is out and I've found the culprit.
One of the freeze plugs had popped out in the head.
Just waiting for my water pump to arrive and I can do a pressure test on the cooling system to see if I need to delve deeper.
Dropped the sump off and apart from a load of milky oil laying in the bottom it looks like the crank is in good condition.
Also managed to free off the completely stuck distributor, given a clean up and back in.
Good weekend so far.