Push button toilet cistern will not stop filling?

How can I repair my push button toilet cistern which will not stop filling. It is a dual flush cistern, about 6 years old. After flushing, the cistern fills, but doesn't stop. Instead it keeps filling and overflows into the toilet bowl. Lifting/dipping the plastic 'float' makes no difference to the flow rate of the incoming water (which can only be stopped by shutting off the supply valve).

Fed up with buying supposedly state of the art stuff which doesn't last.