Oil in Coolant - Already Changed Head, Oil Cooler, etc etc

Hi,

Ive got a 2007 A4 2.0 TDi 170 BRD with 64k. Have oil in the coolant, but no coolant in the oil. Started by changing the obvious - oil cooler, to no avail, sent the cooler back, and fitted another one just in case it was faulty, but within 300 miles the oil was back. Considered head/head gasket, so removed the head, and found very small cracks so replaced it with a brand new one at a cost of £700 for the parts, head gaskets, bolts etc. Put it all back together thinking we'd nailed it. 350 miles later, oil level light came on, checked coolant - oil had returned. Replaced another new genuine VAG oil cooler, and its still coming back. My mechanic is a VAG specialist and works on these engines everyday - were both scratching our heads with this one.

It runs perfectly, goes very well, no running problems at all, except the oil.

Any ideas anyone?

Either the block is cracked or the headgasket has failed. What make head gasket was it?
Did the head bolt holes in the block get cleaned out of any fluid with a air line or a rag and screwdriver.
If the engine has been hot and cold water put in it,this can crack the block and head.