Vauxhall Corsa heater blowing cold

Hi my daughters heater is blowing cold ,not thermostat as the water temp is as it should be ,fan cutting in as should ,water bottle ok, all water pipes hot and going through bulk head,but still blowing cold in the car,(but if you but the setting between the demister and the next setting,(not one or the other, but in the middle of the two )it blows warm,(but only on the drivers side ???,put it back on demist COLD,back in the middle demist and towards you warm but only on the drivers side,(but not as warm as it should be,hope I have explained it so you can understand what I mean,can any one help please thanks tony