Ventilation in a chimney stack

On removing a bedroom radiator and the wallpaper behind, I found a rectangle cut out of the area where the fireplace was. having searched google it seems that an old chimney should be ventalated to prevent damp but this seemed to indicate that this can just be at the bottom which would be the living room. I have a gas fire in fitted which i belive allows for ventalation, is it normal to have a vent half way up? can i fill it? should i just put a vent on? if i dont need it i would presume covering would reduce draft and keep the room warmer Thanks