Firstly the flue must be attached so and be completly sealed the same has your boiler
has it works exactly the same.
Breathing bricks well the idea is there but the way you have been described there function on the
stack is incorrect.
regardless of age of the property, has you keep the internal warm if the building cant breath
then you will produce Damp!
Thats why when you board up the fire place and dont put in a vent and leave the chimney pot un-sealed
the air goes down the chimney as normal and just circulates and as no where to go then produces damp.
What ever you/they do, you must always create a seal and have no carbon monoxide leaks
If you are attaching your flue to a boiler then it needs to be double lined, internal to release Carbon Monoxide and the outer to bring/breath fresh air.
Has long as you seal in and around the top of chimney breast and liner and the liner is connected to the cowl
and connected to the boiler you will be fine and you will not get damp.
You may also need to re-point the chimney breast in your loft has well