Push one knob down ( probably the smaller one ) and lift up the other one [on inside edges, you will actually see arrows] and underneath them is a lage white plastic screw head. Unscrew carefully, so as not to strip slot out of head, and up will come the knobs and surround. You can then lift off cistern cover.
I purchased a replacement from bathroomstudio.co.uk, who also told me how to get lid off.
Keith D
November 2008