Fully reinstate Chimney to its original condition and seal the flue

Chimney flues when originally built would be rendered up inside as work progressed now they are lined with either clay flues or SS liners. Theses methods will seal the flue.

The Flue of a chimney would be built of Brick unless so ething older such as Sandstone . Flues on occasions could have clay liners put in with a Brick surround. These can be used but would need a Sump which is fitted to the bottom if the liner so when swept all the soot etc falls into the sump and easily cleaned to prevent blockage. If no clay liner but just a Brick flue then basically it is advised to have a flexible steel liner dropped down the chimney and correctly sealed and fitted and tested to meet Hetas regulations.