Oil in header tank...

[Charliebucket]

Hi.
I recently checked the levels on my 55 plate connect and discovered a large amount of neat engine oil in the header tank.
It ran fine and there was no trace of water in the oil so I changed the oil cooler as this was advised to be the most likely over head gasket.
I flushed the coolant 3 times and put it all back together but I am still getting small amounts of oil in the header tank. It seems to be coming in through the left small return pipe on the front.
Its nothing like the same amount but I have driven 200 or so miles and cleaned out the heater tank 3 or 4 times since changing the cooler but it is still coming through.

Is this likely to still be oil in the system or am I looking at another problem?