Symptom:
Toilet tank leaks into bowl, causing very frequent (every 15
minutes or so) top ups of the tank.


if there is no leak into the bowel when the tank is full and the
water supply is (nearly) off
but there is a leak when the water supply is on, then that points to a
leaky float valve.


Some float valves have a small hose that leads from the float valve
directly into the overfull drain pipe which suppplies a flow of water
directly into the bowel when the flapper closes but the tank is still
filling.

If you have a small leak in the float valve through this hose, it will
flow right into the bowel and not overfill the tank.

Take the small hose out of the overflow drain and direct it into the
tank. If you have a leak there, now the tank will overfill.

Mark