Why has my van emptied all its oil in the water bottle?

I have a 51 plate ford courier van 1.8 td di the other day whilst i was driving it the temp light came on so i stopped and looked under the bonnet to see the water bottle and surroundings covered in oil, the water bottle was also full of oil, the van was driving perfectly and never lost any oil or water or had foam in oil cap or any tell tale signs of head gasket, i have had the oil cooler off to test by blocking one pipe off and blowing in the other in a bucket of water but no bubbles, completely at a loss as to what it is or what to do next, any help would be hugely appreciated.