It is essential that chimneys are built correctly, following tried and tested methods and structures for several reasons.
Firstly, it is important for any fire or stove that air flows through it quickly and efficiently in order for the fuel to burn cleanly to produce the desired heat. This means that any chimney must have good air flow.
Secondly the chimney stack must be built following UK Building Regulations. This is to ensure that the chimney stack is sound and unlikely to collapse due to poor construction. If the chimney stack does not conform, it could invalidate your insurance in the event of a problem.