it sounds like the cistern infill valve is not cutting off. Back int'day, toilets used to have an extra external pipe that took overflow out through the wall into the drain - now cisterns have this built in, so if your cistern is over-filling then the excess water drains into the toilet bowl.
The regulator of he infill valve needs to be altered. This is usually a screw-like plastic gubbins on one side of the inside workings of the cistern.
If you don't feel confident about takig the lid off the cistern, bear in mind that this is a five-minute job so don't let anyone overcharge you to do it.



