Apologies, I know similar questions have been posted before but as a non-diy person this problem is starting to get me down, here we go... Two water tanks in the loft, large and small, each with their own overflow pipe to the rear of the property, both the ball cock things appear to shut the water off and I have bent the arms slightly to get better closing pressure. I took several bucket loads of water out of of the small tank as it was overflowing and went away for the weekend, the central heating and hot water were on via the timer. On our return the small tank was full again, well over the ball cock and water coming out of the overflow. How/why is water entering the small tank, what could be causing this? Although it did not happen on this occasion I believe that the large tank also fills up past the ball cock and out of the overflow but this reduces when taps are opened, again why would this be? Central heating boiler is five years old, the rest of the system is approximately ten years old. Any advise would be appreciated, thank you.