I've had the same problem as you just this week & have isolated the water entering the cistern (luckily I have a downstairs toilet too) until the plumber arrives, hopefully soon..they seem very busy people! I didn't know I had a problem with water running into the bowl as it wasn't really noticable & it was only when I got my recent water bill which had doubled in price for the same period last year, I realised something was wrong & after speaking to my neighbour, she told me they had the same problem a year or so ago & told me to check it. So, if your water bill has increased, you will know why