All sorted. Cut off the white collar with a junior hacksaw, without cutting the grey cable actuator. Pull the old push button assembly from the actuator. Fit the new push button assembly into the cistern lid and simply push the grey actuator
onto the button assembly. But be warned, once pushed on it locks and the only way to get it off is to buy a new push button and go through the above procedure again. :-)
George Williams
January 2015