1. Try turning down the water pressure to the cistern.
and if ball float type
2 Cut a poystyrene tile to fit under the ball float stick it to the ball with mastic sealant. Ensure the tile does not obstruct/catch on the side of the cistern or any other part inside. This sometimes stops the 'bouncing' which causes the banging. Yes the tile works I've tried it on a main tank ball valve float.
bob, October 2009
