Assuming everything else is in order the most likely cause of your lack of heat is an air lock, not an uncommon problem.
The best way to cure this is as follows.
Turn both heater knobs to full.
You now need to disconnect the heater flow pipe from the engine.
This pipe is the one that rises off the top of the head near the back.
Undo the jubilee clip and pull the heater hose off the connection to the head. Get some one to place their finger over the metal pipe end while you hold the rubber heater hose vertical and fill it from a jug with coolant.When full quickly reconnect rubber hose to head connection whilst coolant is coming out. Tighten jubilee clip and hopefully you should have a working heater.
If not , it's not air locked.
To avoid air locking problems it's always worth doing this proceedure after any disturbance of the cooling system eg fitting new thermostat etc.