"I would guess used to house a real fire" You need to find out for sure. If it is a fairly modern house, it may only be a feature not intended to have a real fire. Real fireplaces are lined with fire bricks and on a concrete platform (I'm sure there will be a real builders term for that).
I know of someone that decided to have a real fire in a feature fireplace and ended up with 3 fire engines at the door... Speak to a competent builder.
Thanks for the replies.. pretty sure it was a real fire, it's an old house (circa 1920) with two proper chimneys, one in the living room, and one in the dining room, although the dining room fireplace has been blocked up.
You can also feel the draft coming down the living room chimney, probably needs a sweep though.
Was mainly worried about the size of the hole. All fireplaces I have researched are too large, and don't really want to get builders to extend the hole to fit a fireplace. (Can't really afford that at the moment)
I had a similar problem last year, I found a small stand alone electric coal effect heater fitted, quite small but just a little wider than your space although mine stands forward slightly of the rear hole due to being a bit too wide.
You may find that the hole was larger and has been bricked up at some time to be used as a flue for a gas fire. If you can get a mirror in the hole you should be able to see if there is a lintel at about 3ft above the hearth, that would be the original hight of an open fire place, that is the mantelpiece.