Overflow

The water tank in the attic has a problem.

The ballcock is completely submerged in water which is constantly filling up the tank (although not sure how as the ballcock seems to be at highest point therefore not opening valve) and the water is continueosly emptying via the overflow pipe.

I have been suggested of a fault in which the ballcock has developed a hole and is now full of water and sinking slightly allowing an opening in the valve although having checked this out it is not the case.

Any suggestions?

Thanks in advance.

The float is either leaking and no longer totally floating (thus it can't move high enough to close the valve) or the valve that is controlled by the float needs repair and is no longer shutting the water off.

It should be fairly obvious. Pull up on the float to see if the valve turns off. If it does, adjust or probably replace the float. If it doesn't, repair or replace the valve. Often, a new valve would also contain a float. You can probably find a replacement float if the valve is okay.