After I have flushed my toilet, and after it has refilled itself, water keeps slowly running down into the pan. It isn't a gret deal of water but there is always a constant noise there too.
I'm about to start renting out my flat and want to get it fixed. Any ideas what it could be?
You could read the articles on this site to gain a better understanding of how a toilet works. You may have a flapper that needs replaced, your water level in the tank could be too high allowing it to run over the overflow tube and/or your refill tube could be inside the overflow tube resulting in siphoning. Remove the lid on the tank and observe it's operation. It may be a value to replace the fill valve with a Fluidmaster 400A and to replace your flapper.