New masonry chimneys, UK stove building regs J
In Document J, page 22 & 33
New masonry chimneys should be made with flue liners and components suitable for the application.
You can use pumice, clay, or concrete liners.
Manufacturers guidelines must be followed and the space between the liners and surrounding masonry filled with a weak insulating concrete backfill - ie leca, vermiculite, or perlite (unless manufacturers guidelines say otherwise).
We recommend never using vermiculite, use leca instead. Vermiculite is quite soft and compresses if it gets damp or wet, leaving the top of your chimney uninsulated.
The joints should be kept to a minimum and manufacturers offset components used if there needs to be an offset in the system.
Our advice: for a stove we would only recommend using pumice liners. We consider that clay or concrete liners are only really suitable for an open fire, but even for this application pumice would be better because its a more insulating material and therefore much less likely to lead to condensation in the chimney and will also make your chimney work better.