Heritage toilet leaking water into back of pan?

I purchased a Heritage bathroom suite 2 years ago and after 9 months water started leaking into the pan. Heritage sent out new parts for inside the cistern but after 4 months it started leaking again. I have since had a Heritage plumber refit these parts twice and after 3 days and 1 day respectively the same problem occurs. My water pressure has been checked. What is causing this, I am on a water meter so have to turn off the water every time after use. I am at my wits end. Can someone help?