50p says there is an old back-boiler pipe that is leaking.
Somebody needs to hack up the concrete to look.
A general builder can do it.
If not, you may need to dig out the old fireplace and find the DPC, and lay a new plastic DPM before refilling.
The old chimney must be ventilated to prevent condensation. If you are digging out the hearth you may be able to put in a duct so that air from under the floor goes up the chimney. This will prevent draughts in the room, and help ventilate the subfloor as well.
The fireplace in the dining room was filled in by previous owners. The area where the hearth would be was filled with concrete. We've not touched it, except to have a specialist render over the top when we had the damp proof course in that room redone a year ago.
There now appears to be moisture leaching into the hearth/concrete from where the fireplace was. The plaster above is not damp - just the floor. It's spread out about five centimetres and has grown visibly larger in the last week.
I'm a bit baffled as it hasn't really been raining that much, and we've never seen it before. It's a party wall, so I guess worth going round to have a word with the neighbours. It's likely to be the chimney though, isn't it? .
Do I need a general builder, or someone more specialist?