The ballcock almost certainly is worn and needs to be replaced. Loads of info if you Google ‘replace a ballcock’. Take off the cistern lid and locate the ball which is attached to an arm, the end of which blocks the inflow when the ball floats upwards on the filling water. It’ll make sense when you see it! You can usually test for a worn valve by putting slight upward pressure on the arm when the cistern is full. The inflow should stop. As the cistern is very old you’d be as well replacing the entire apparatus including ball and arm.
Here’s the hard bit. The inflow has to be stopped while the valve is removed. I do it by placing a rod across the cold-water tank in the attic and tieing its ballcock arm up to it. I then lower the water level in the tank below the outflow point. There may be a better way but it works for me!
The cistern’s ballcock is removed by unscrewing a large nut around the inflow pipe on the outside of the cistern. The arm/ball is attached by a split-pin the ends of which are turned sideways to prevent it from coming out. A screwdriver should enable you to straighten the turned ends and a pliers will remove the pin – it can be a bit tricky.
The new parts are available at any plumbing supplies’ outlet. Install the new parts and release the ball in the attic-tank. Check that the water level in the cistern doesn’t get too high. The ball is screwed onto the end of the arm and there is some adjustment possible. In the worst case the arm may have to be bent so that the ball is lower but this must be done very carefully!! Good luck!
