Blowers/Heaters

Hi all, 

So I have been experiencing some intermittent issues with the heaters/blowers in the car, Its a 2011 Mk2 VRS Diesel.

When the temp was set to low It was fine for most of the time blowing nice cold air in to the cabin, then on random intervals with out changing anything it would just start to pump hot air in to the cabin.

This was with out Air Con turned on, I could turn off the blowers and turn it back on a couple of times and it would go back cold.  However now its unusable its always blowing hot air in and even with the blowers turned fully off I still get hot air pushed in to the cabin through the vents.  This is much more obvious on long runs or motorway driving.

Could anyone offer any advice on what the issue is?

Thanks