There is a water valve fitted in the heating pipes between the engine bay and the scuttle, roughly in the middle is a black plastic section with two pipes running through it with a electrical connection. This valve opperates to let your engine warm up quicker before letting hot water get to the heater. These valve do occasionaly stop working and dont allow any hot water to the heater matrix which in turn will make your heater blow cold. I would have a look at the valve and proberly replace the valve if it isnt working
