I had the same problem. If its like my dual flush, there is a screw underneath the 2 flush buttons. Once you unscrew that, you can remove the cistern lid.
The best way to access the screw is to de-press one of the dual flush buttons all the way down, apply pressure to the side of that button with the same finger you used to press it (so you're kind of pushing it against the side) and pull it up - the screw will be revelaed. OR one other way i found by accident, is to run your hoover over the top of the cistern, the suction draws up the flush buttons and you can grab one that way!
Christina, October 2007