Massimo Liguori
Are you sure you have a sealed hot water system? From what you are suggesting, i.e. that your hot water cylinder expansion pipe is dripping into your cold water tank, you are describing an open-vented hot water system, where the vent pipe is discharging into the cold water tank. If you have a sealed or unvented hot water cylinder, you would not have a cold water tank in the loft, since the cold supply to the cylinder would be at mains pressure (reduced to an appropriate pressure via a pressure reducing valve). Thus, if you do have an open-vented hot water cylinder, water would travel up the vent pipe when the water being heated in the hot water cylinder expands. With a correctly installed and set system, it is unlikely that the volume of expanded water would expand into the cold water tank. Is the cylinder thermostat working? If not, it could be that the cylinder water is boiling which would cause it to expand into the loft.