cold water tank

[JoeyB]

Why does the tank overflow run

[MORELLO]

Probably because your ballcock is knackered, allowing too much water to gather in the tank. Relatively small and easy job to fix, - beware of rip off merchants.

[inicholson]

It does just sound like the ball ****. It could be one of the seals but they're not worth faffing round with nowadays.

Replacing the whole ball **** is an easy diy job. Shut off your water, disconnect the old one, and take it to any DIY or Plumbers Merchants and they will give you a replacement. It'll cost as little as a fiver.