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I opened out the fireplace to reveal the builder's opening, hohum. The opening is inset into the wall, and the wall is plastered with normal gypsum plaster to the edge, albeit ragged at the edges now due to me opening out the fireplace. Were I to get the plaster tidied up at the edges, would it survive a stove?

The opening is 23" wide, the stove is 17" wide, and I can hold a hand at that distance from the stove, so not red hot. I am concerned the plaster would crack with time, due to the heat from the stove. A plasterer said it might well crack around the lintel, above the stove where the heat rises, and he suggested fireboarding the wall around the opening, and using Easi-fill to blend in with the wall. He would do that, not me. Thoughts please!

Hi Leif,
If your plastering near the stove you need to use a lime mix plaster or something like Victas heat resistant plaster.
But do what Ive done to mine, I have similar sized opening and clearances as you, I used 18mm fire proof board ( might have been supalux ) and stuck it directly to the bickwork with Sika-Flex then sprayed it matt black, I also cut out and used it for the register plate.
Been okay for 6yr now.

He mentioned heat resistant plaster/render, but said it was a pig to work with. He suggested lime render inside the opening, but I think I'll leave it as brick, though render could look nice.

I guess you have confirmed that gypsum plaster near the stove is no good. I know plaster inside the opening is a no no, but I was not sure about the wall near the opening. I think I will ask him to fireboard the wall.