This is the older mechanism with the rectangular "card", as you descrip it, captive in a rectangular slot at the top of the mechanism. The lid is screwed to this and forcing it off would have broken it as it required the buttons removing first and the screw under them undoing. However this part regularly failed and the latest (its also the same for Armitage Shanks as same company) version dosen't screw down the cistern lid. I've attempted to obtain one of these parts but, as is usual these days, its not available. You will therefore unfortunately have to replace the whole flush mechanism which isn't too difficult.
Stephen
December 2009
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