We also built the frame around the twin-walled chimney. This will be enclosed with magnesium board. It is important to keep all combustible materials at least 50mm from the chimney, and to maintain a 50mm air gap around the chimney. This boxing will achieve both these things. When we put the magnesium board on, we will need to ensure that we put in air vents to allow any heat to dissipate.
Insulation and rafters in the roof also must be kept 50mm away from the chimney.