Fitting a Clay Fireback Advice Please


Can anyone give me some tips on fitting a clay fireback? We've opened up a wall where a fireplace used to be, but there's just a hole with no fireback.
I assume I need to back-fill the void between the back of the fireback and the back wall and make good any gaps on the sides, but ISTR fire cement coming into the picture somewhere - also, what's the deal with the optional lintel they also sell - how do you know if you need to fit one?
No one at the merchants I went to seem to have a real top on the answers, so I thought I'd pick your collective brains. I've tried Google, but I haven't found any detailed fitting guide.
Any pointers gratefully received.
TIA
Steve
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You also need a surround and a hearth. Theses fits in front of the fire back. Place the fire back on the hearth and flush with the front of the brick opening with equal gap each side. if the back of the hearth is not filled in level into the brick opening, this must be done. place the surround on top of the hearth and up to the wall, Offer the surround up to the wall and opening. Pull forward the fire back up to the back of the surround. Remove the surround away from the wall, so you can now get to the rear of the fire back The gap at the back and sides of the fire back as to be filled in. Some do it with brick and some just fill with lime mortar or a weak mix of sand cement and brick rubbish. The top is sloped off towards the back of the chimney flue at about 40 -45 degs. Make sure that the fire back hasn't moved forwards. Offer the surround back to the wall as secure with the tabs and screw to the wall. Point any gaps between the surround and the fire back sides. The lintel is a smoke and flame defector, But I cant remember how its used.Some uses them some don't. Re plaster around the surround to stop smoke seeping out from behind the surround top and sides. That's all I can remember, it's about 50 years ago since I've seen one put in.
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Thanks for such comprehensive, step-by-step instructions Keith - exactly what I needed. I'm very grateful to you - absolutely brilliant.
Best
Steve
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