Marble Fireplace

That Dave, depends on how it was glued there in the first place.

Which is as long as a piece of string.

One thing you cant do is use brick acid to dissolve plaster or mortar:-(

And marble can and does snap.

Normally there's a metal lug on each side which allows screws to be used to hold it back to the wall, normally around 4 to 12 inches down fronm the top on each side is where you'll find them but they are probably below the level of the plaster. Hopefully it'll just be these holding it back but it's likely that some form of adhesive has also been used, probably tile, but possibly foam or even silicone...you may have some luck with an old bow saw blade.

Expect plaster damage

Victorian marble fireplaces are usually constructed of many individual pieces of marble, glued together with Plaster of Paris. It was probably assembled close to final position on the floor. P of P poured in the void at the back of the columns, then erected against the wall. There will probably be a pair of lugs 2/3rds of the way up each side either clout nailed or screwed into a mortar joint then plastered over. If it hasn't previously been removed and re-fixed then the original insert may have been broken up in-situ. Can you not carefully trim the extremities of the replacement insert to fit into what space you have. A 1mm cutting disk on an angle grinder copes well with thin cast iron.

AWEM

You generally find they are secured along the top twice and once on each side with a metal plate and screw.. If you chip away the plaster you will find it. You might be able to detect the position with a nail finder if you have one.

This is right - I have lived in several houses all pre 1900 and the fireplaces have been "glued together with plaster of paris and fixed to the wall with wires under the plaster. Dig out the plaster for an inch all round and it may come out whole - bit often they come apart at the joins. You buy old marble fireplaces in bits.

I find a small rare earth magnet more precise

That's a good tip, a lurker says thanks.

Yes - it works well on ceilings looking for the joists (by locating the nails).

I don't know, where would I get a rare earth magnet?

Andy

And if not, you can get some pretty nice fridge magnets which will clamp 30 sheets thick to any bit of tinplate.