Heater / Aircon Only Kicks Out Cold Air on sub-zero mornings

Hi, I have 2005 Peugeot 307CC 2.0 litre 16V

The last two icy days this month the heater on my car has only kicked out ice cold air despite the Climate Control settings being set to 25degC or more - I tried setting in both Auto and Manual settings but no warmth came out of the heater. Also pulled over a couple of times, turned off the ignition and restarted the engine to see if it would kick it into life.

Outside temperature was initially -1degC rising to 3degC and the Engine Temp was normal. After an hour of travelling the heater still did not kick out any heat and arrived at work cold and miserable. After an hour restarted the car and once the engine warmed up, the heater was working once more.

The first time this happened I put it down to a one off anomally, but now happened twice I'm worried about travelling into work on a cold and icy day for fear of suffering from hypothermia!

Any idea what the fault may be?

Thanks
Neil