Read the stove manufacturer's instructions.
Not all aspects of fitting an inset are intuitive.
For example, on the one I installed the instructions explicitly stated that an air gap must be left between the stove side walls and the opening.
This seemed a bit strange since the side walls of the stove formed the air pocket that was warmed by the combustion chamber.
I had thought of filling it with vermiculite based mortar - soft and crumbly but good insulation value.
I phoned the manufacturers to confirm their requirement and they were adamant that a gap must be left and not filled with any form of insulation or soft mortar.
I actually took out the outer wall skin and did the flue fitment and connection from the back side. This saved having to dismantle the existing mantle piece etc and caused minimum interior mess. Once everything was working satisfactorily it was an easy job to refit the outer stonework.
