How to deal with this hole?

[PuraVida]

We've had our back boiler removed. Eventually we'd like a wood burner but can't afford it yet.

I need to get the surround replastered soon

I'd thought to remove the modern brick infill, which is supported with a metal lintel as per the pic. But I'd expected there to be an arch above - the original supporting structure

As you can see there is just one course of bricks laid vertically. Presumably this is the original opening.

If I knock out the newer infill is my entire chinmney going to find tumbling down?

I know I need professional advice but I can't get anyone out begged xmas and I want to get the plastering done before that

Halp!

above the modern fill, there appears to be a very rough, slightly arched or joggled course of bricks on end, including the black mark where the original builders might have had a fire in the builders opening.

Bricks have a natural tendency to arch themselves, especially once the wall is a foot or two high above the hole, even if a few fall out. The hole will tend to resolve itself into an equilateral triangle at the top if this happens.

You can take out the lintel and filling, the chimneybreast will not fall down. If you are worried about it cracking, you could put needles in while you are working.

The original build was always intended to leave a big hole (the builder's opening) which somebody would put a hearth and fireplace into. As it was never intended to be seen the workmanship is very rough and there is a chance old bricks might fall out if disturbed.

You ought to finish the hole to suit your future stove before you have it plastered. If you want the opening to be any higher than the original builders opening you ought to support it with needles and put in a new lintel.

Or to delay spend, you could leave the existing lintel in place and plaster it, in the knowledge that it will have to be opened up, resized and replastered later.

You will need to board up the chimney to reduce heatloss (but with an airbrick-sized ventilator) or it will get damp from the inside. Unused chimneys do that, unless ventilated top and bottom. You can put shelves or a cupboard inside the hole for now. Dust, birds and bits of mortar may fall down.

Thanks PJ that's helpful.

The sooty end on bricks are not arched, they're a straight line.

Luckily the chimney is full of the old liner, filthy insulation and dead birds so we're leaving that be for now