Megane II Cooling Problem

I was driving my Megane and noticed that the heater was blowing cold air despite the engine temp showing normal, suddenly the temp gauge shot up and the display read engine overheating.. The car went into the garage and they fitted a new thermostat as it had stopped working, they tested the car and said the heater was now blowing hot air and was ok to drive.. I drove off but I have noticed that the heater is still blowing cold air and only gets warm when the temp gauge drops to the quarter mark and then suddenly warm air starts to come through and the temp gauge goes back to the half way mark again..

My question is could this be a blockage in the heater matrix?



Thanks for looking