Hi i have a a4 b6 1.8t my heater only blowing cold ive things ive tried flushing heater matrix new thermostat new water pump bleed system heater will worm up if i drive car hard but soon as i stop gos cold again any help be brilliant

 

have a 2005 A4 Avant B7 and only blows cool air when on high setting, this started today.

Could it be an air lock as it's just had a new cylinder head a couple of weeks ago, if it is how do you get rid of an air lock ?.