back to wall toilet keep leaking water after flushing

[magalan]

I have a back to wall toilet that keeps leaking water into the bowl after flushing. When I mess around with the plastic bag it stops..but will start up again a few days later.. any ideas what i need to do to stop it complete

[alavapint]

The vast majority of cisterns have their backs to the wall ... post a picture of the inside of the cistern, this would help a lot .. !