What to do with this fireplace?

[DecorDilemma]

I'm in the process of redecorating my living room (1930s detached house), by stripping everything back and pretty much starting again. The fireplace previously contained a gas fire, which was disconnected and removed a while back - one of the photos shows how it looked at that point.

I recently removed the fire surround (carefully, and in one piece), so I could take off the ugly "inner" bit, which I have now discarded, and see what I'm dealing with. You can see in the other photos what is left. It looks like breeze blocks and an extra layer of bricks were put in to accommodate the gas fire, but I'm intending to keep the fireplace as purely decorative, so I think I want to remove them and open it back out to the original size.

I guess I'm just looking for opinions on what's going on here/what I should do with it. I also wanted to check which part is the lintel. Is it the bit directly above the black fireplace hood? I'm also wondering if that's the original lintel, or if the fireplace was messed with when the gas fire was put in. Ideally I'd be able to strip back all the plaster so I can see properly, but I'm not sure if this is something I can do myself. I'm expecting to get a builder in to do the bulk of the work, but I like to get as much information as I can in advance so I know exactly what to ask for.

I would have painted the original wooden fire surround and tiled on top of the 'ugly' beige insert that you've taken out. I've done this before. The tiles stuck fine and it was a quick fix.

@snotathing I'm not necessarily looking for a "quick fix" though, and the insert was cracked, heavy, and loose, so not a good basis for tiling over. FWIW I also plan to change the hearth. It looks like there were various bits patched in to accommodate the gas fire, and I'd rather not have that as the starting point for a new design. I thought opening it out could look nicer?

We are having a similar thing, with 2 of them.
It looks like it does have a lintel, but the builder will know, I'd be interested to hear what they say. One of ours has a rough thing very much like it, we found it when we unbricked the bricked-up centre part, it seems pretty sound so we're leaving it at that. The same hearth also has a bit of brickwork in front of the flue back, probably for an even earlier electric fire, as we're going electric we'll just leave it there behind the fire. The other fire is much the same, but with a deeper central section, which we'll probably build a new one in front of, possibly pseudo-Victorian with tiles. That flue has been lined for a gas fire, we may have to get that out,we'll be taking advice on that from the sweep. Both chimneys are capped, the gas one when we did the roof (we had the fire out before, of course) and the other had been capped before we came here.
We are getting a sweep in to clean both chimneys as far as possible before we finally fit the fires and the new surround edges round the new fires, because we can't be sure it was done when the chimney 1 was blocked and lined and/or the other was bricked up. There is some soot around, but not a lot.
If those side walls aren't in the way of what you want to put in, it might be best just to leave them there, again, the builder should advise.

The lintel is probably above the bit you have currently uncovered. If you carefully remove the plaster above where the hole is, you should find a brick arch. Once you can see how it has supported, you will know how much of the fireplace below you can knock out. The metal cowl won't be structural so that can be removed.

That surround is beautiful. I would be arranging my whole room around it.

It’s quite possibly jarrah, aka Swan River Mahogany, exported from SW Western Australia by the boatload.