How can I fix my running toilet cistern?

When I flush the toilet (Ideal Standard cistern, 4 years old), when the it fills up again it flushes, this may happen a number of time before it stops, every time I use it, the number of time it flushes on its own increases, I am unalbe to remove the top of the cistern, because of the push button.

I understand that the unit screws off and it should be able to be dome using the only fingers, however the push button is ceased, I have gripped it using a pair of plyers and turned the unit, but the whole button unit turns around on the cistern lid, not unscrews, the screw ring on the unit appears to be ceased in some way.
Please advise.