Does your actuator have a small bump on the top? If so you can replace the ball and O ring by removing the actuator and using the above kit. If you do not have this bump, you will have to use the 40003918-007 kit to replace the ball; it costs about £10-£15.
It will have the bump on sir as he has already replaced it, I think replacing the ball only is a false economy as it will be getting clogged with dirt and or scale.