I had a similar problem with my potterton profile 50e. The header tank outlet was blocked, when I opened the bleed screw on the pump nothing came out. I completely cleaned the tank and cleaned the pipework which had pulled a load of sludge (from the bottom of the tank) into the system. Once I got water at the pump I could hear the air in the system being pushed around. I bled all rads (took a while) and it did the trick. Make sure your header tank is clean and full. Hope this helps.
Phil, January 2008
