How can I get the cistern top off myt Gala cistern?

My Gala cistern has only one button and all the answers so far are quoting 2 buttons.
I cannot for the life of me see how to get the************* top off.
I am no idiot , I am an atomic physisist and taught Einstein and Hawking all they knew but this cistern will not reveal its secrets to me .
Is there another Genius out there or are you too ashamed to admit defeat>