I had the same problem with a Villeroy & Boch 2 button cistern.
The solution (which I found on this web site) and [hopefully will work for you) was as follows:
Depress and hold down the half flush button.
There is an indentation on the full flush button which should now be visible. This will allow you to use a screwdriver to prise the FFB up and out. Remove the HFB
You should then see a screw which when undone will allow you to remove the cistern lid.
Hope this helps
James Mortimer
June 2009