Oil in expansion tank

Hi I have a K12 1.5 DCI diesel. Yesterday I turned the car on and heard what sounded like running water. Just as the engine started I put my head near the glove box and the noise was like water running out of a tap at very low pressure (like a small gurgle). This stopped after about 10 seconds.

I checked the water and the expansion tank was low, but more worryingly there was oil in it. But this isn't emulsified oil. It is black oil. There is no water or emulsification anywhere on the diptstick, nor under oil filler cap. There are no compression problems, There is no white smoke out the exhaust, the car is not over-heating, There is no build up of pressure in the expansion tank after a run.

Could this be anything other than a Head Gasket Failure?