Whats causing my peugeot expert to overheat?

Hi someone please help my 1999 Peugeot Expert Van 1.9 turbo diesel keeps overheating I have removed the thermostat replaced the radiator bypassed the heater matrix just incase that was blocked but it is still overheating and causing massive pressure build up in expansion tank which causes it to spit its water out of the cap I have no oil in coolant or coolant in oil van starts and drives fine until it overheats could it be head, head gasket or water pump HELLLLLP!!!!!