I am fitting a closed wood burner and need to remove the existing
fireplace and need some advice.

There is a 1040x1040mm cast iron fireplace with 500mm wide fuel holder
suspended 150mm off the hearth, above an ash box. I want to remove all
this part. It all seems to be in a single piece of ironwork but there
are no visible fasteners, unless they are hidden by the handful of
Victorian tiles that decorate the sides. This ironwork is surrounded
by a 1740x1260mm marble mantle with pillars each side which I want to
leave as is.

I am wary of starting the job with a big hammer because I would like
to salvage the original fireplace and I don't want to take the risk
that some critical part is holding up a part of the chimney stack
There's a lot of house above me to fall down..

Any suggestions for me?