heater blowing cold on my 2004 corsa?

In a previous thread I was advised to check for air blockages. I have massaged the pipes with the lid off the header tank and the fan worked fine, blowing hot air, the temp gauge also read the correct temp. However, when the car was then driven the temp gauge went back to zero and the air started blowing cold again. Any other suggestions please.

Dave L
March 2009