Firstly feel the heater pipes, as they go from the under bonnet area to the insdie of the car/heater box.
These should be nice and hot, if they are then the problem is the heater box, there are several things it could be, though if you can get hot air anywhere it shows the cold/hot control is working, so that leaves the divertor flap, this controls where the air goes, ie floor/screen, vents etc.
Get your head under the dash around the heater area and see that when the controls are operated it is causing a flap/arm etc to move, a cable may have come detached.
The normal problems with cold running stem from a knackered thermostat in which case the engine will not warm fully to temp, and be sluggish getting any kind of temp, or on some fords they have a water tap(controlled by the hot/cold control) on the bulkhead area that interuppts the supply of heated water from the engine to the heater unit.
tabetha