I did just the same when we moved here.
Ours was a large copper cylinder set into the fireplace. Jemmy it out,
(and weigh it in), then you fill in the holes with firebricks/bits of
fire bricks with lime cement.
You then put you fireback into position, it may need lifting up etc,
then fill the back, again with a mix of firebrick, or, plain lime
cement/mortar.I think there may have been some refractory insulation in
there too.
Let that go off, then fill all gaps with mortar.
There may be bits I've missed as it was 15 years ago when I did mine.
I got a book from the library that showed exactly how it was done
(surely there is a step by step website now?), and it has lasted well,
the fireback has started to wear out a little at the bottom, otherwise,
the chimney sweep says it is all good.
Alan.
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