Turn off the tap feeding the cystern. Flush the toilet.Take the lid off the cystern. Remove the "L" shaped pin that holds the float arm to the valve mechanism by rotating it 90 degrees then pull the pin out. Lift out the float arm. Completely unscrew (anti-clockwise) the cap on top of the valve stem & lift the little valve body from the top of the stem. This item has a flat rubber ring attached (remove & replace) & also a small seal (11 mm diameter) inserted in the end. Remove this and replace. Re-assemble etc.
Trevor Freeman
March 2009