This is the state of my fireplace:



As you can see it is the old style firebrick insert, and the old hearth and surround are now gone. I could put in a slabbed granite hearth and back panel (the n-shaped bit that sits on the wall around the hole). But I like the idea of removing the firebrick insert, and the loose stuff behind it, to open up the fireplace into a chamber. I am told it would be safe to burn logs once opened out, though I would not do this often, I am more concerned with something that looks nice. Are there likely to be any problems with this? I assume there is a structural lintel somewhere above the opening, so it is safe to remove the firebrick.

Once/if done, what would I repair the edges of the opening with? Gypsum plaster? Would I put some beading round, then slap on plaster?

It depends a lot on what material you have behind the fire back, as some types of brick/block can blow/shatter through to much heat.
The fire back you have at present is correct for open fire, as its throated to enable proper smoke draw.
You dont want plaster board or plaster any where near the fire.
Best option is to build a fire surround in brick/ stone of your choice.

Thanks. I think I'll stick to the granite hearth and back panel rather than open it out. I'm not sure I like the idea of brick, although I will have a think about that.

Leif......part J of building regs may help with info.

Not easy to understand though, not for a non builder person such as myself anyway. From a quick look, it looks like opening out a fireplace is not trivial, as far as regs go. So I am coming to the conclusion that the easiest safest apparoach is to leave the firebrick in place (it is compliant with regs), fill small cracks with fire cement, then add a hearth, back panel and surround. I have toyed with the idea of a tiled hearth and back panel (granite tiles, 1cm thick), but I am unsure about how to do this, though I might ask in the tiling forum. And it looks as if cutting granite tiles is a tough job requiring serious cutting equipment, which I do not have. I guess a pre-made hearth and back panel is the way to go.

Our granite chappie uses a 4.5inch angle grinder with a super dooper diamond blade for granite and it eats through it.