On a roof tank you should find the tap on the top pipe.
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.
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.
There should be no water gushing out.
They work by rising with the water level in the tank which in turn activates a lever which then shuts the valve.
Ballcocks also known as float valves are found in water tanks and are used to control the flow of water into the tank.
Just follow this video it s easy save money on expensive plumbers.