WATER FROM CISTERN CONTINUES TO RUN INTO BOWL?

I HAVE A B&Q PORTO UNIT WHICH HAS STARTED TO CONTINUE RUNNING WATER INTO THE BOWL AFTER FLUSHING.
I HAVE REMOVED THE LID AND CAN SEE 2 UNITS WHICH APPEAR TO BE FUNCTIONING CORRECTLY. TO STOP THE FLOW I HAVE TURNED THE OUTSIDE SCREW INLET PIPE OFF. THE FUNNY THING IS AFTER A WHILE IT STILL ALLOWS WATER IN BUT DOES NOT LEAK INTO PAN. HELP!