We have a modern toilet with dual-flush press buttons on top of the cistern. When you flush it, it is now "running through" more often than not. By running through I mean that, after the initial flush, water continues to flow through to the bowl and the cistern continues to fill but never makes it, due to the escape of water. This goes on until you lift the lid and waggle things around. Sometimes just lifting the lid can cure the problem. We are concerned about this as we are on a water meter. I can't work out which part needs adjusting or replacing. Any help appreciated.