i agree with fordtec, it sounds like the heater valve.
you could test it by removing it and replacing it with two small bits of copper pipe the heater will then be on all the time even if you select cold on your controls it will still come out warm (of course you need the engine warm before it comes out warm)
the heater valve is just in front of the windscreen wiper mechanism if you look at the pic below the red circle is on the two pipes that connect to the heater valve and the other side of the valve you have another two pipes that go into the bulk head of the car to the heater matrix.
just be careful when removing the pipes that none of them breaks the plastic pipes on the heater valve because the plastic goes brittle over time, this is what i done and i had a bit of plastic floating about in the coolant system which kept blocking the heater valve and caused overheating.
http://www.fordforums.com/attachment...1&d=1192664082