Technical Booklet L Combustion Appliances and Fuel Storage Systems·Page 34·2.45
Dry lining around fireplace openings
Gaps in dry lining or decorative treatments around fireplaces must be sealed to prevent flue gas escaping into voids, using materials that can accommodate movement between the treatment and fireplace recess.
Where a decorative treatment, such as a fireplace surround, masonry cladding or dry lining, is provided around a fireplace opening, any gaps that could allow flue gas to escape from the fireplace opening into the void behind the decorative treatment should be sealed to prevent such leakage. The sealing material should be capable of remaining in place despite any relative movement between the decorative treatment and the fireplace recess.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet L - Combustion Appliances and Fuel Storage Systems.pdf