toilet cistern constantly running?

I had an Ideal Standard toilet fitted in Feb 09 and my plumber is still finding it impossible to stop water running into the pan constantly. It is not the overflow, and he has just replaced the whole cistern and insides with no improvement. Is this a design fault with these dual flush button ones and would I have been better to get the lever handle type? I am on a water meter so have had to switch the toilet water off. It is a Ceate Edge toilet.