I had same coolant problem on my x type 2.0 diesel 56 plate. It was a rubber o ring.
I took off air filter and the bracket below that it mounts on.
Under that you'll see the water pump, on left hand side of the pump about 1" from the block is a plastic connector held on by an 8mm bolt.
I undid the bolt and removed the plastic connector, the o ring around the plastic connector was broken. It seems to be some kind of sub thermostat with a spring loaded valve. Part number XS7Q 6L635-AE
Hope that helps someone.
Mick
