your problem is the heater control valve thats mounted on the bulk head under the bonnet
Open the bonnet remove the plastic guard just under the windscreen and in the middle you will see a black unit where the heater hoses go into/come out of
you need to remove this unit be sliding the clips from the hose's and removing the hoses then remove the untit from the car ,get a new heater control valve or try a second hand one but these may not last .As you have the unit out its a good idea to flush out your heater system ,get a hose and feed water into one of the houses that are coming out of the bulk head ,you should get a lot of rusty smelly water pouring out the other end ,keep the water going in till its cleat out the other side . Fit you new heater control unit making sure your put the control on you dash in the cold postion (blue) once fitted start the car up let it warm up then turn the control to hot and it should blow nice warm air .
It a bit of a time consuming job and can be a bit cold in this weather and a bit sore on the fingers getting the hoses on/off but if worth it to have a warm car .
Dave
December 2008