Inside the cistern there should be a water level mark about half way up either on the side or back. when water stops filling the cistern see what the level is. IF well below this mark as it sounds, then at the water inlet end of the float arm is a nut and threaded bar the end of the bar pushed agains the valve to close off the water. Loosen the nut and turn the threaded bar in the direction so then valve end moves away from the valve and the float raises hight when gently lifted. Give it about 3 full turns then before re-tightening the nut see if the level reaches the mark. If not keep turning the adjustmen bar a bit at a time until it's right. then lock down the nut.
