hello all, i have a focus 53 plate tdci, there seems to be a issue with overheating, its pushing water back into the expansion tank i then have to turnn the car off as the needle will go into the red, after undoing the yellow cap for the expansion tank and waiting some time the water normally goes back down it will then drive ok at around 60 mph and some times the heater works and other times its cold, ive put a new thermostate in and a new water pump, so next step must be head gasket? ive just tryed some K seal but this has done nothing, theres no oil in the water and no water in the oil, what do ya think guys deff head gasket?

thnaks
andy