Just realised I missed your comment about the disappointing survey.
The likely reason your installer will not fit a certain size stove into the recess is that manufacturer's request a certain air gap left and right of the stove (often 6"). This air gap is there for two reasons:
- To ensure that the heat from the stove gets to the room and is not just heating up the inside of the recess.
- To ensure the recess does not overheat.
There is no building regulation involved here - a stove can go as close to a brick wall as you like as long as there are no combustible materials around (inset stoves almost touch the brickwork). You cannot overheat a brick or stone.
Manufacturer's instructions override Building Regulations but only if it is a safety issue.
As long as your recess is made of non combustible materials then there is no safety issue involved - it is purely about the heat getting to the room (a stove crammed in to a tight recess might be a disappointment).
One way of fitting a stove in a small recess is to choose a stove with a 45 degree rear connector. This means that the flue pipe comes off the rear of the stove (stove can be swept through the stove by the way but it must be a 45 degree connector and not a T connector).
Because the pipe is at the rear of the stove a goof proportion of the stove is "in the room" and half in the recess (a couple of offset elbows in the vertical flue can bring the stove further forward into the room if required).
This is a useful method if the stove wants a 6" air gap and the gap is actually 4" for example. Because a good part of the stove is in the room rather than the recess then there is no issue with the heat not getting to the room. I would happily do this for a customer as long as customer is happy. Of course a part of the rear of the stove is 4" not 6" air gap but we are using common sense here and there is no reason for any concern whatsoever.
The hearth does NOT have be 86cm x 86cm in this case (that is for freestanding stoves where no fireplace recess exists). As long as you have a minimum 22.5cm of hearth in front of the stove doors you are okay (30cm best practice but 22.5cm min building regs).
Charnwood range of stoves (C4,5,6 etc.) all have 45 degree adaptors for this purpose.
Now some stoves only request a 3" air gap. Convector stoves might also be worth looking at. These are twin skinned so the outside of the stop does not get very hot (the heat comes from the glass and air vents in the stove) and therefore a manufacturer might specify a smaller air gap.
Google Julian the Stovefitter if you need to find me for advice...