Chimney closure

I have a detached house with a chimney dead centre of the property to originally serve a bolier in the kitchen. As I have a new gas boiler now on an outside wall in the Kitchen I am using the old boiler alcove as an extra kitchen cupboard and have filled in the hole in the ceiling of the cupboard where the flue went up. I have also had the chimney taken down off the roof. I am now attacking the chimney that goes through the loft space I want to get it down to under the level of the floor boarding in the loft that rests on 4"x2" joists. My plan was to cement a thick flag stone over the chimney hole level with the floor boarding in the loft to make it safe and level. Now you have the background my question - is it necessary to have an air brick in the remaining chimney wall in the loft before I cap it off under the flag stone. The chimney now is about 8' 8" long with none of its sides on outside walls. What do you experts think please