UK Car Repair: rover 75, blown head gasket, combustion gas

Question
MY 2 LITRE V6 ROVER 75 is chucking water out from the expansion tank overflow all the time as if its overheating or boiling up yet my temp gauge is not going over the half mark ? please have you any idea why & also where is the thermostat on a rover 75 v6 2 litre ?

Answer
The thermostat is in the lower rad hose BUT I doubt that is the problem as the gauge is not registering overheating! It sound as if you have a blown head gasket, the combustion pressure being pushed into a cooling channel. There is a tester which can check for combustion gas in the coolant, perhaps you could find one or have a garage test it!