The overflow pipe from the tank in the loft is leaking - a lot. My property has been emply for the past three months while I've had a new kitchen and bathroom put in. During this period the central heating, hot water and electricity have all been shut off. The water stop cock has also been shut off. After the builders left I turned the water on and the overflow started to drip at an alarming rate. This morning I had the builder and his plumber back in and they replaced the ball cock in the tank in the loft. This did not stop the overflow from dripping so the plumber said I needed a new hot water tank which would cost me £350 fitted. I had a second plumber in this afternoon and he said the ball cock was set too high. He readjusted the arm and took £10 and left. The overflow is still leaking. What can possibly be wrong?