Further help on removing the old push-button unit.
On many types, people are confused as to how to remove the push-button unit - you can't get the lid of the cistern off because it is held down by the (silver) ring around the two push-buttons, and there's no obvious way to remove the buttons. The secret is to push one button down, then you can lift the other button right up and out - its on a stalk. Repeat for second button, then the silver ring can be removed. You will almost certainly have to purchase this unit as a whole.