Replacing the valve will stop the leak, but it won't cure the problem.
The split is caused by a shear force on the valve shank. When the plastic ages it becomes more brittle, then splits.
The shear force results from a poor installation - the copper tail isn't aligned with the cistern. If it used to be aligned, it will never stay that way because things move. The remedy is to cut a larger hole in the tiled surface so that the pipe can move slightly.
