heater (blowing cold) not working on Ford Escort Mk 5?

when I put heater on, no heat only cold air blows through, even when the engine has reached it's working temperature. the hose to the thermostat housing is hot and the top one leading to the heater unit behind the bulkhead is also hot but the lower one is luke warm. I replaced the thermostat approx 1 year ago and have tested it out of the car with boiling water and it opens ok, all the controls seem to activate ok and the blower is working ok. any ideas?