Vauxhall Astra-H 2007 Oil In Coolant Issue

I bought this Astra (Z18XER) in from a trader who said it needed head gasket done, after looking at it I realized it was more likely leaking from oil cooler as engine oil had emptied and coolant was full of emulsified engine oil. No emulsified oil in crank case at all, no over heating, no signs of head gasket issue in cylinders or spark plugs. I removed exhaust manifold and oil filter housing with attached cooler, when I separated it from from the housing I could clearly see emulsified oil in coolant and clean black engine oil at inlet and outlets of cooler, I put it all back together with new cooler and gaskets, it was all good for several miles but has now got the exact same fault. I am guessing it is still not the head gasket (due to the obvious oil transfer occurring at the cooler) but more likely the oil filter housing which may have cracked internally from the heat of the exhaust manifold. Does anyone know if this is a known common fault with the housing, I have replaced these in the past on customers cars but always with cooler and housing the problem usually being just oil leaks rather than transfer of oil into coolant!