Before you go any further check that there is no water in the ballcock float.
Roof tank ballcock.
How to change the ball valve ballcock in your cold water storage tank in the loft.
There should be no water gushing out.
Ballcocks also known as float valves are found in water tanks and are used to control the flow of water into the tank.
On a roof tank you should find the tap on the top pipe.
One of the most common toilet repairs is adjusting or replacing the fixture s fill valve.
Just follow this video it s easy save money on expensive plumbers.
This part is often known by the generic term ballcock although strictly speaking a ballcock is an older form of this valve that is rarely installed anymore.