Usually the two push buttons need to be removed. Press one down & take the other out & then remove the other button. Usually there is a black screw (large) undo this & the lid should come off. The inside should have a central column turn this gently( can't remember which direction) but be careful as these can brake easily. Remove & turn upside down there will be a large washer at the bottom if it is undamaged just remove it wipe it over and turn over and replace in the column. This should extend the life for a while but if you need to replace the whole thing Roca are about 20 quid for the whole thing.
Sparticus
February 2009