I have a boilermate II linked to a potterton Suprima boiler. The heat exchanger in the potterton went and the full system was drained to change the heat exchanger. Everything went fine till I tried to refill the system when it would not fill. Had to eventually fill the radiators back through the drain tap. Anyone have any idea why the boiler mate will not fill through the header tank. Have tried everything re air locks etc that I can think of but to no avail. Any help would be appreciated.