Remove the lid of the cistern - the automatic fill valve needs to be adjusted to lower the water level - operate the flush and observe the action - how the float rises and through a lever shuts off the incoming water - your float should be lowered slightly - the water should typically shut off about half an inch below the overflow pipe level.
If there's a small nylon locking nut be sure to do it up else you'll get another overflow before too long.
Good Luck..., December 2015
