How do you remove the lid of a Roca cistern with a pull flush?

The chrome knob in the centre of the cistern lid has a rod running out of the bottom. This rod goes through a hole in the lid which is smaller than the chrome knob, and is attached to the mechanism inside the cistern. There is no obvious way to remove the chrome knob from the rod, so what to do? The cistern is allowing metered water to run continuously into the toilet bowl, so any ideas would be much appreciated.|||try the company website http://www.roca.com/|||I have no idea, but good question!