(HELP) needed expansion tank over flowing?

hi my central heating system is over flowing water through the expansion pipe going to the tank in the loft when the heating and water are on together you can feel hot water flowing up the expansion pipe in to the header tank then drips ouside through the pipe but when i knock the water off and just run the heating it doesnt do it has anyone any idea what it could be have turned the pump down but still does it been told that it mite be the coil in the hot water tank but dont know how to check for this any ideas will be appreceated thanks daz