Do you have 2 water tanks in your loft/attic? and is the smaller of the 2 installed higher above the larger tank, if so does the small tank keep filling via the ball valve, if so the small tank could be filling the large tank because the coil in your hot water tank(cylinder) could be fractured, never seen it before but possible.
If this is the case I would stop using the hot water for bathing or cooking etc. would love a response
Bored Plumber
August 2009