You may well find that curved steel goes right through the sides of the breast and angles down/up on the outside face of the side bricks under the plaster, in effect securing the fireplace against spread. Looks just like ours and this was how ours was done.
Cool, I certainly hope so that would be nice and secure.
Not sure if I am going to expose the vertical chimney breast bricks up to the arch yet and may just have the arch showing which seems to be in good order so far. The plaster is pretty thick in some places (2 cm) but I can probably get a magnet to test that.