My cold water tank is overflowing.  Somehow warm water is getting into the cold water tank and causing the water level to rise and overflow.  It's not the ball valve, nor is it coing from the immersion heater expansion pipe.  I think it's somehow coming back up the pipe that feeds the water heater.  I think it might be something to do with one of our mixer taps.  Please can anyone explain how to fix this.  Is this just a question of replacing the mixer (if it is that)?  Any help would be much appreciated!