probably the cooling unit itself, had the same problem on mine, replaced the cooler, twas about £130 and when i took the old one off the gasket was fine, you should get a new gasket with the cooler so you might aswell change the gasket too. give the area a good clean before you put it back on. oh yeh, and starter motor needs to come out if its the td4. about an hour or so job. it will take some flushing to completely remove all the old oil. im on about my 4th flush this weekend and still have oil appearing, but its getting less and less.
Try dishwasher liquid, citric acid, any degreasant.
I removed the header tank and put petrol init and didnt even need shaking, it was spotless. was tempted to flush the system with unleaded (bad idea i know) but i know for a fact every last drop of oil will be gone.