At worse it will be a crack in the cistern which will have to be replaced. However it`s worth lifting the lid and checking inside the cistern to see where the level of water is. If it`s high and overflowing into the overflow pipe , then it means that the ballcock arm (that controls the incoming water) will require adjusting(usually be bending the arm downwards) If it`s not that , then the only other thing is to attempt to tighten up (don`t overdo it !) all visible compression joints just in case it`s a joint that is leaking....Still failing , then call the plumber: Useful tip: If you have a job finding where the water is leaking from, dry everything with a dry towel then try to locate the leak using COLOURED toilet paper. This shows up any water straightaway.
Nobby
June 2008