Noticed the same thing happening today! I turn the heater switch from Hot to cold and it takes a good couple of minutes before it finaly starts to cool. if on the other hand I switch from cold to hot it works pretty much straight away?
think of it as a radiator in your car,when you turn your heating of at home it takes a while for the radiators to cool down,the same thing applies to a cars heater matrix it wont go cold straight away....
its probably an actuator flap problem! probably motor that moves hot/cold flap gets stuck in some point
the HCV controls the flow of water to the heater matrix, 99% of problems with heater staying hot is due to this,replace only with a genuine ford and upto date version....