Upstairs chimney breast brickwork construction

tph216

I've recently bought an Edwardian mid-terrace house, that has fireplaces in the front and rear rooms on ground and first floor. In the upstairs bedrooms, the two central / inner alcoves were largely boxed-out and plastered over, with shelves built in lower down.

I've started removing the boxing-in, to open out the alcoves back to the brick chimney breasts. These come in & begin to gather in the first floor bedrooms, and finally join in the loft before going into the stack which is central on the roof apex.

I've got most of the plaster & stud-work off in one of the bedrooms, and can see up onto the underside vee where the two brickwork flues begin to join. I also see though, a single thickness brick arch, stood off/away from the party wall, spanning between the two breasts. It is stood off from the party wall about a bricks width, and there is nothing supported above it (I can reach round it with my arm and run my hand along the top of the arch).

I cannot think what its function is, any ideas?

I'm not thinking of taking it out, but there is a wooden stud-work arch under the brick arch, onto with the laths were nailed. As far as I can tell the brick arch isn't in contact with the wood arch (i.e. isn't being supported by it). I am intending to take the wood / stud-work arch out, and then lime-plaster everything back up again, right to the brickwork, so that I have a bigger, more usable pair of alcoves in the front and rear bedrooms.

Also, there was a noticeable amount of soot at the bottom of the stud-work boxing-out I removed, but as far as I can see there are no holes in the chimney. It was extremely fine soot, and I assume represented 112 years' worth of build-up, so perhaps wouldn't need much of a leak and may just have come from small air/smoke leaks over the years?

If anyone knows anything about this particular construction detail (and its purpose), I would be keen to hear. Not much seems to be turning up on Google. I don't even know what that area of the chimney structure is called technically.

Thanks in advance for any advice / help.