check that the pump hasn't failed (ie, put a towel underneath to catch any water and protect eclectrics in boiler) remove the centre screw from the pump, you should only get a few drops of water and more and the pump bearings may be worn, the when its on, put the heand of a screw driver to the inside wall, if the pump is turning it should clatter the screwdriver about.
Replace pump head with new.
T, May 2008