A constantly running toilet cistern could signal a number of different problems with your toilet. The cistern (as it is commonly called in the UK and parts of Europe) keeps the water supply for the bowl prior to the flush. A fill valve refills the toilet after a flush, while a flush valve allows the water out of the cistern. You don’t have to wait for a repairman to figure out how to repair a cistern. There are a few common problems that happen with cisterns frequently.
Place a towel on the floor underneath the cistern to catch any water that may run out.
Screw the bolts on the bottom of the tank tight with a wrench to stop a leaking tank nut. Use a screwdriver to hold the bolts steady in the tank as you tighten, or have an assistant hold it for you. If that doesn't stop the leak, check the stoppers on the top of the bolts as they can start to come apart after a time underwater. Replace if needed.