Hi, I had a similar problem some time ago on a customers system and at first could not find any leak to justify the slow loss of water. Eventually I found that there was a leak in the heating coil inside the Hot water cylinder. This corrosion would have been caused by electrolysis, and although possible to change just the coil, I found it more economic to fit a new cylinder. Problem solved for my customer.
May be the answer to your problem as well!
Good luck.
Mike
November 2007