Get the thermo out the housing first Dan and check it by hanging in some boiling water from cold and see if it opens or does anything or take it into the with you.

Also flush the whole system, it could be an air lock somewhere? Try getting the engine to a warm/hot temperature with the heater switched to hot and feel the 2 pipes that go thru the bulk head, see if their getting hot at all? If they aint its 99% an air lock.

Has the wire broken on the twisty knob for the heater?

Failing all that, is there enough air in your tyres? and is there a Vanillaroma magic tree hanging from your mirror? All this could lead to the heater matrix going.................