Our builder recently (Mid 2009) installed a Kohler dual flush toilet in our new bathroom. No model number is shown from what I can see.

Water is now constantly dribbling from the cystern into the bowl but I cannot take the lid off to find out what is wrong. I'm assuming that I have to undo the chrome ring surrounding the dual flush 'buttons' but I cannot do so.
Is there some special tool to undo this ring, and, assuming I DO manage to remove the lid, can anyone then advise me what to do to stop the water constantly running into the bowl?

Any help gratefully received! Thanks!