Did you not check the stove manufacturer’s requirements before you bought it?
Have you contacted the manufacturer for their advice? As
Hotrod said, the manufacturer’s installation instructions will always take precedence & if you ignore them, you will probably invalidate your stove warranty but are also likely to invalidate your house insurance if there is a problem associated with it on which you subsequently make a claim.
The stove is not designed to be inset & the gaps are obviously there to provide air circulation. Without the recommended gaps
a) the stove may overheat possibly causing damage to the stove
b) the walls surrounding the stove will get rather hotter which again, may cause damage to the surrounding render & even the brickwork.
What did the HETAS registered installers advise? A builder’s opinion is irrelevant & if you go the non registered/LABC certification route, I cannot imagine any BI going against MI's & certainly not against regulations. I’d be very suspicious & if you go down the non registered builder route, make sure he actually makes a Building Notice submission (or better still do the submission yourself) & make sure a BI is actually involved; some builders ignore BR’s & tell lies about BN submissions. Please be careful what you do next.
Having said all that, the fire opening could possibly be opened out depending on construction, sometimes there is also an inner lining course of brickwork making it easier still. I assume you’re referring to backfilling the flue liner; if so then I advise you do it (loose backfill is sufficient), it helps maintain flue temperature through the stack giving better draught, stove efficiency & keeps flue gas temperatures up reducing acidic condensation near the top of the flue.
