It's probably due to the flow valve not closing off properly when the cistern is full of water. You can try adjusting the nut adjacent to the flow valve to increase or reduce the flow of water. The float should rise to the highest position and this effectively shuts off the flow valve into the cistern. You might need to replace the valve if there is damage - these are about £5 in B and Q and are easy to fit. However, it doesn't always solve the problem and you might need to experiment with the float level to get a complete close-off.
Gary
January 2011
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