Ray is quite right ,the WD40 does work .Although on my flush the smaller level indicator on the side of the unit was the trouble as it was sticking and preventing the plunger from going down far enough, you can ensure it is operating ok by making sure the that the bottom of the plunger is down a good 1 ins from the unit casing, if not it means the plunger is still sticking and the water will still run thru it into the toilet pan,so just make sure its 1 ins .
The direct replacement for the unit if you decide to change it is the "Kayla DVA 0100" ,at the moment its about £15 on e-bay, or the "Fluidmaster 550073 or Fluidmaster Pro 05502.
My unit was difficult to get out of the cistern, but when i gave it a really hard twist ,which was maybe a bit more than a 1/4 turn it popped out easier,its easy peasy to get it back into its cradle that stays in the cistern.
Rob, October 2014