And if I could manage to stem the leak from this overflow pipe with some
tcp tape, should I not worry about the water level?

Talk about curing the symptom rather than the disease!

You need to work out why the valve hasn't turned off the water before the
level gets too high. Blocking up the overflow is not the solution!

Does the ball valve work? Does it stick on something? Has the ball get a
leak and taken on water? Has the washer worn through?

Christian.