Same problem, with our Roca dual flush cistern.
I spoke to Roca sales support (01530 830080). They told me: depress one button and prise the other out carefully. Then prise out the second button, revealing the black screw that fixes the lid on.
The also said they would email me a product care/maintenance leaflet, which contains this kind of information. Wonder why the plumber didn't leave us this if it was included with the unit when it was fitted??
Andrew, November 2008