hi, my cold water tank over flows at night the ball cock is in good condition washer replaced and no water seems to be entering through the feed but the water is not cold and warm water seems to be entering through the bottom feed? i have recently fitted a new bath and basin with new pipe work for a metre or so.
I am no expert but i think you should check your new pipework as it sounds as if you have connected the cold into the hot at some point which could push hot water back up from your cylinder to the tank.
The only other thing i could think of is if your system is overheating and boiling.
Surprised that you have not had more definite replies to this post.