I have a single flush button toilet and the flush had become very weak ( ie only a small amount of water comes out ). I wanted to have look in the system but the top is tightly secured and wont move. I have seen how you get the dual buttons off - but this is a single button which I am struggling to remove ( have tried to screw to no avail ). Please could anybody tell me how I can get the lid off and then what the problem may be ? Thanks