that jumble of old bricks has been used to fill up the hole when the original fireback was removed. They should pull out fairly easily, and you will find properly laid bricks in the original, rectangular "builder's opening"
it looks to me like a gas fire might have been previously fitted, venting up a slot near the top.
Soot, rubble and bird skeletons will fall down, so cover your carpet and furnishings.
Your should have your chimney swept and inspected before putting back into use, it might need a liner, and it may have cracks or holes allowing smoke and fumes to escape.
A modern clean-burning multifuel stove is much better than a woodburner (which you may be prohibited from using).
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