Hi, if your toilet flush valve is similar to the one shown in the video then you will see how easy they are to replace if your toilet is constantly flushing. If you only have a small amount of water letting by into the bowl then you may just have a bit of debris which can be easily cleaned, or it is easy to replace the rubber washer. My one in the video, must have been caused by scale build up from the hard water inside the flush valve so it was easier for me to just to replace it for under £10.
Many thanks Vince.