Having scanned dozens of front doors in the neighbourhood to get an idea of what other people have by way of ventilation, I concluded that our circumstanced seemed fairly unique (couldn't really find other doors with the same type of set up).
As you can see there's a 7" panel at the bottom of the door frame. My assumption was, behind the panel, would be the floor joists and sub floor.
With help from my Dad we removed the panel in the hope of installing the air vents.
To my surprise instead of exposing a sub floor, we discovered that the hallway floor was in fact slightly raised, beneath which was the original stone tiles (not sure why the previous owner decided to cover it).
Having cleared the rubble we began to cut out the door panel to fit the air vents.