The efficiency of ancient boilers like your late back boiler is in the range 50 to 60%, but I think that is before allowing for the draughts through the massive hole. Old gas fires can be as low as 30%.
Where you need a b*** great ventilation hole for an appliance which is not the main heat source the efficiency could be negative, i.e. the heat loss thru the hole 24/7 is more than the heat you get from occasionally running the appliance. You could close the hole when the appliance is not running, but if you forget to open it when the appliance is on you could possibly die from the fumes; that is why Bldg Regs require the hole to be non closable. It is also one reason why all modern boilers are balanced flue, i.e. they have their own built in air intake, so no separate vent is needed. A few wood burning stoves have the option of an attached air vent.