I followed this thread and flushed the radiator three times before realising I should open the heater to flush the heater core too. Then I replaced the Auto Transmission filter and gasket.
It now looks like the problem was caused by loose inlet manifold bolts and by tightening these bolts I have squashed the gasket back down and might have solved the problem. It has improved but still some traces of oil in the radiator. Sludge is gone.
One more flush of the radiator with heater fully open, fingers crossed might clean it up.
Here is a reference pic of the inlet manifold bolt locations.