Usually water leaking Jon-stop is due to valve faulty.
Just take a photo of the valve system in the cistern. Go to a shop that sells toiletry stuff and buy a similar valve unit. Be sure to buy one of similar height as these valves comes in different sizes.
Turn off the tap supplying water into the cistern, Remove the cistern where it holds the water, change the clave, put back the cistern. Just a couple of plastic screws to remove and put back.
As simple as that. No brainier.
Not sure where you are, dyi cost is about one-fifth of what you pay if you get a plumber.