TOILET CISTERN OVERFLOW
Overflows here are caused by a failure of the ball valve washer controlling the water flow.
Most probably your problem valve is an old one. In this case the replacement washer will be rubber, in an Imperial size —almost certainly ‘A in. (to match the supply pipe).
1. Now disconnect the ball arm from the valve by removing the split pin and then juggling the end free of the washer piston.
2. With the arm removed. Unscrew the piston cover.
3. Stick a nail in the slot where the ball arm entered the valve and flick the washer piston out through the end of the valve.
4. On the other end of the piston is the washer. Dig it out, unscrew the collar, and fit a new one.