Technical Booklet L Combustion Appliances and Fuel Storage Systems·Page 60·4.20
Flue outlet guard protection
Flue outlets must be protected with a guard where a person could easily contact it or where it could be damaged.
A flue outlet should be protected with a guard if – (a) a person could easily come into contact with it; and (b) it could be damaged.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet L - Combustion Appliances and Fuel Storage Systems.pdf