I've replaced the thermostat, and another problem is the heater in the car is blowing cold, cheers everybody

drain out ur radiator n add in radiator flush,fill up the balance with water.start ur engine 4 30mins.then drain out the radiator water again.u w'd see the water is brown color.clean the radiator with water then u add in the radiator coolant n top up the balance with water.ur car now should be ok on the road.if not there w'd be a leakage in ur engine block,u've to relap ur engine block surface n change the top n bottom gaskets as well as valves grinding.