mk2 golf gti heater not working

[Diamond Hell]

Flush the heater matrix and the radiator with a hose.

Your thermostat might be stuck open.  What's the engine temperature doing?

[OakeyDoak16v]

Try a new/different matrix?

[john905]

have replaced thermostat already,when i was bleeding it up, bottom rad pipe always stayed cooler, then the thermostat opened and the pipe got hot and fan cut in so i know thats working fine temp gauge reads 1/4 driving and when stopped it goes to 1/2

[JC]

airlock maybe    had this after a stat change .

re- bled the system ( good guide in the MkII section ) and jobs a good 'un 

[jamie_pyrite]

Replacing the water pump and thermostat sorted my heating out!

So glad it wasn't the *spits* heater matrix!