639 vito Coolant tank pressurising

Hi all
The expansansion tank on my 2005 vito 115cdi seems to be pressurising as it loses coolant, but only on a long run, and its definately out of the overflow.
My first instinct and worse case scanario is the head gasket but so far I have no water in the oil or oil in the water and no overheating or white smoke on first start up.
Also it doesn't need much to top up the tank maybe less than a litre each time, vito has just done 130k miles.
Guess I already know the answer but has anyone got any other theory's or experience of this with the newer shape vito?
Any positive help would certainly be appreciated