Thanks to JohnD
You can buy a ball-valve repair kit, but as a new one is so cheap it is hardly worth the trouble and better to renew.
If you undo the blue nut on the RHS, you can take the other parts off the stem and swap the new one for your old one. The old plastic ball float screws onto the end. After taking off the old and before fitting the new valve, turn the water on for a few seconds (with the lid on to reduce splashing) to flush out any debris from the pipes.
The white plastic screw on the RHS should press against the side of the cistern as its purpose is to prevent strain on the threaded shank of the bottom-fed inlet valve.
The white plastic screw on the arm is to adjust the water level.
