what is wrong?
overflow is leaking at odd times during day and night.
ballcock and valve on offending tank checked and appear all ok. it seems as if a backflow of water is coming into tank from whatever it is supplying.
i've had centrall heating and hot water off for 48 hrs and problem still occurs.water level in tank rises about five inches(125mm) HELP!!!

John D Walters, July 2009

if your talking about the cws tank that supplies your hw cylinder.

it sounds like your getting backflow from mains pressure cold to gravity hot.

shut your main stopcock and see if it stops
if it does thats your problem.

hi-spec plumbing & heating (essex), July 2009