On a trip up home from Kent to Yorkshire last weekend, I noticed after about an hour that the car was getting quite cold inside even though I had been slowly upping the temperatures. I noticed that the volume of air coming from the vents was minimal, but was quite hot. Even setting the temp to HI and maximum fan just a trickle of air was coming out, not ideal when it is chucking it down and the windows are starting to steam up!

I have noticed this once or twice before in the summer when, again on a journey of over 40 mins or more the vents would stop blowing cold air. A quick press of the ECON button and a few minutes later the airflow would return allowing me to go back to Auto and the cold air would once again flow in.
Back to this weekend, I decided to pull over and feed the family in case it was something more serious and we were stuck on the side of a cold wet A1. So when I returned to the car, I could hear the vent controls making some noise and again the heat would flow out much to my relief and again the heat ran for 40 mins. I did the summer AC trick of hitting ECON and we were kept warm for the other two hour part of the trip.
Anyone got any ideas of what part of the system is playing up? I will pop out and do a Vagcom scan but that hasn't picked anything up before.
R