Is it Armitage Shanks? If so:
Press and hold the small button.
Remove the large button by popping out with a screwdriver.
Pull out the small button.
Undo the large screw underneath the buttons.
Remove the button housing.
Lift off the cistern lid.
I've just done it and it is easy.
Hope this helps.
Jill
Jill T
February 2010