cistern?

the toilet is a heritage dual flush. I am having problems with the cistern it will not stop filling. I have checked the "cup" thing which appears to regulate the volume and rate of water re fill and even when in the highest position it does not stop the water from filling. There is a a long mesh net like tube which comes out of a tube next to the device does this have anyhting to do with it as there appears to be a hole in it