1997 Vauxhall Astra water pressure problem

have you bled all of the air out of the cooling system after topping up coolant,to bleed the air out start with the engine cold and top up the coolant level,start the car up and switch the heater to hot and let the engine run for a few minutes with the expansion cap off to let the air out,squeeze the hoses to help the water circulate,once the heater starts blowing warm air then put the expansion cap on and run car up to normal temp.if the water does`nt circulate then you have a restriction or blockage in the cooling system.try flushing the cooling system through when cooled down,hope this helps