2003 Vauxhall Corsa heating not working

take the rad cap off run the engine for a while to make sure the water is curculating could be one of two things the thermostat or a blocked heater matrix check thermostat first

There may be air lock in the cooling system, take the readiator cap off and start the car, let it run on idle to normal operating temperature, you'll see air bubles in the water and the water level will rise and then go down, turn the heater on at full speed and keep topping up with water untill there's no air bubbles in the water.