No warm air on drivers side...

Just been out for a spin in my car and noticed that the screen and footwell were blowing warm air on the passenger side but cold air on the drivers side. I was wondering if anyone had any ideas what the problem could be.

The heaters take a while to get going normally but once upto temp they are normally very warm. I don't think that the drivers side was receiving any warm air at all. The heaters definately worked a few months ago and I think they were OK last time I used the car - about 3 days ago.