fireplace wall render/plaster

I'm renovating a traditional fireplace and have discovered the render to be sand/cement which I'm pretty certain isn't original. Question is, what was there previously in a 1930's semi and what should I put back there? Would browning & finish be okay, or should it be sand/cement? In my reading I haven't seen this mentioned. Fireplace will be used with 10kw input living flame gas fire.
TIA

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Sand/cement/waterproofer may have been used because the brickwork behind is too damp for gypsom plaster directly. This can happen because of lack of damp proof course in the rubble/soil used to form the base of the fireplace and hearth, which breaches the damp proof course in the brickwork. This didn't matter when the houses were built as a regular fire kept it dry, but becomes a problem when the fireplace is no longer regularly used (and I doubt your living flame fire will send enough heat into the floor to keep it dry).

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A-ha I've replaced the constructional hearth with a new DPM'd version. So, so long as the heat isn't going to affect the plaster, I'll plaster it.
Thanks, Mike W

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What about the floor inside the fireplace?
I have done a couple where I have removed the hearth (taken it down well below floor level), floor-boarded across the top, and then built a new hearth on the floor. For the fireplace base, I dug out down to the level of the damp-proof slate in the brickwork, lined the hole with DPM, filled bottom half with vermaculite, and the top with sand and cement. I'm not actually using this as a fireplace, but it could be in the future.
Plaster is remarkably heat resistant (I've had it glowing red hot from a blowlamp without apparent ill effect), but I wouldn't use it _inside_ a working fireplace. Should be no problem on the chimney breast though. I still used a sand/cement/lime/waterproofer scratch coat though for the bottom metre of wall, as in my case the outside ground level is a bit high, and I'm not going to do anything about that.

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You mean the back hearth? The constructional hearth is all one piece with DPM overlapping a good bit above the DPC. I shall bonding & finish it like elsewhere. Fire & hotbox came yesterday. Vermiculite tomorrow. Cast iron combo & hearth on Saturday, ooh exciting. Thanks, Mike

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Before WW2 lime was used after ww2 cement was used so it was probably replastered after the war, maybe the fireplace isn't that original?
I'd use browning or bonding as you suggest.

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Yeah when I said traditional fireplace, I meant standard 28" builders opening and class 1 chimney. There is no fireplace, I'm putting one in.
Stay tuned fireplace folk for, no doubt, other related questions.
Thanks, Mike