water is filling the header & expansion tank via the feed pipe to the pump?

Over the last few weeks I have noticed that the overflow pipe to the header tank has been dripping, but I have proved that the water is coming into the tank via the outlet pipe (I've emptied the tank and dried it, then watched the water slowly fill the tank from the bottom outlet pipe) The tank outlet pipe feeds to the pump - a Grundfos Selectric UPS 15/50.Is there anything I can do to rectify this problem or do I need a new pump? Although the heating system is currently off I did put it 'on' & bleed all the radiators and I have checked that the pump rotates freely. The system seems to work very well other than the problem described.