Just follow this video it s easy save money on expensive plumbers.
Roof tank ballcock.
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.
On a roof tank you should find the tap on the top pipe.
Before you go any further check that there is no water in the ballcock float.
How to change the ball valve ballcock in your cold water storage tank in the loft.
Ballcocks also known as float valves are found in water tanks and are used to control the flow of water into the tank.
There should be no water gushing out.
Make sure the water is turned off by taking off the lid of the cistern or tank and pushing down on the ballcock arm.