You'll have to excuse my ignornace of all things toilet related, I'll try to explain as best I can.
We have one of those toilets with two buttons as the flusher on top of the cistern. When you press them down the loo flushes and the button pop back up. The cistern then fills back up. At the moment the button pops back up but it's loose, so the water comes into the cistern but then just runs straigh through to the toilet bowl, as if it flushing really slowly. The only way to stop it is to keep pressing the button down until it pops up properly. When I came in on Monday it had been left running like that all weekend by someone! I'm sure something just needs tightening somwhere, but what?
It'll be overflowing into the pan, as opposed to the older design that ran through a pipe into the garden.
It probably needs taking apart and refitting to stop it sticking, would be my guess. If it isn't obvious how to do that when opening the lid, then maybe best bet is to get a plumber to look at it. Can't be that expensive, surely.
I had the same problem.Plumber came and replaced the mechanism and cost 30 quid.Mind you the plumbr charged "mates rates" so I don't know what the full cost would be.