You said you are not getting hot air into the cabin. Is the fan blowing cold air? My pollen filter was filthy and it made a big difference to heater temperature once I changed it.
Curiously it seemed to highlight a stuck open stat also. Before I changed my Pollen filter, my car always head up to about the middle of the temp gauge. Once I changed my pollen filter I noticed it took longer for the temp gauge to reach midway - highlighting a stuck open stat. I changed the stat and now it heats up to middle of guage within 5 mins driving.
The 2 things shouldn't really be related. I can only assume that lack of air flow through pollen filter took away the cooling effect of air flow through the heater matrix.