diesel in the water

After having a new engine installed in the disco I have found a new problem which so far has baffled all the local mechanics.

I have a problem with diesel getting into the water system and filling the expansion tank.

I have by passed the fuel cooler thinking it could be the source of the problem. No.

The only thing that it can be is a cracked head. Any other ideas would be appreciated.

is it a td5? you could well be looking at a cracked head if you are getting fuel in the water