my mate had similar thing on his 200 tdi, i used a running hose pipe through his heater matrix, i undid the rubber hose near the stat and also undid the rubber hose off the end of the metal pipe that runs back along the head to the front of the engine, i let it run for a good minuit then re conected the pipe back to the metal pipe let it run some more then pull the hose pipe off and reconect the first pipe this should shift the air in the matrix, its to fidderly to do it at the back of the engine, take it for a good spin with the header tank cap off.. this made things a good 50% better on his, but he was still not happy so we replaced his hole heater box now its 100% good luck..