Many thanks for your advice Richard. I have contacted a HETAS installer and they say that they can do everything including the building work, so it sounds like I might be safer leaving it to them.
I am planning to install a woodburning stove in my property. I have no existing chimney so I need to form a new fireplace recess (approx 800mm wide x 800mm high)in the external cavity wall between the house and the attached garage. The stove will sit in the recess with a small masonry chamber (2...