You will find that if you push one of the buttons down and lever the other up with a screw driver, it will pop out of the housing. You then pop the other out by pushing it up and this will reveal a plastic screw head. Undo the screw and lift up the cistern lid complete once the scerw is unscrewed. Thus revealing your cistern workings. You'll find that the leaking is due to the diaphram in the cistern needing replacement.
Beecham
January 2009