It looks a bit odd for a new build; you can almost imagine that it was only ever intended for decoration. Does the chimney breast extend into the room, or is it built onto the outside as it's got a very deep recess. The stove is only 355mm wide, and is designed to fit into a standard 16" opening, and as you're not going to get a smaller stove, I'm a little perplexed as to what the new installers are going to suggest, so I'd be a little wary of what they suggest.
When I suggested you widen the opening, it's a case of a builder taking a row of bricks out of each side of it, as I think the stoves okay, and just needs a couple of 45 bends to bring it further into the room, but you need a builder to adjust the chimney opening.
As the initial installers aren't Hetas registered, they should at least take the stove back and give a refund, and you might agree to accept a reduced cost for the liner.
I am willing to believe that some of these stove fans work, but my BIL bought what looked like a pretty good one. He curses it and claims that he would get more heat from the cat breathing on his hand.
I have one, not a thermocouple type but a Sterling engine model, which does blow warm air out into the room very effectively and becomes more powerful the hotter the stove. I suppose it's all down to good design.
A bit more expensive at about £80 I think, but it is very well made and maintenance free. Not even any need to lubricate as it uses a graphite piston. To be honest, I bought it partly because I was interested to see first hand how it works.