One single patch of damp in bedroom of house built in 1901

Hello for this kind of problems means water is getting in eaither through the brickwork mortar or cracks, to stop this you will need to know if it’s lime or sand and cement, and don’t mix the two because lime is a breathable material and to remove the stain you will have to sand down to plaster treat the mould and then re paint and that should do it

Dulux oil base undercoat apply to area plus 6" past area allow to dry 24 hours then 2 coats of emulsion then a knowledgeable inspection of roof loft ceiling etc it will be something so simple if the right person assess situation best of luck

Is it on the ceiling or walls ? Sometimes I find that if it’s on the ceiling there could be a cold spot (where cold air it getting in thru the soffit and facia) there’s a possibility I could be as simple as checking your loft to ensure your insulation is adequate in the area where your damp it showing, ensure you don’t fully open windows as the cold air mixed with the warm air in your house will encourage mould and try to Ava our drying clothes/ towels on radiator to lower the moisture vapour in the room, If this isn’t the case then I suggest further investigations to where you have water entering your property and rectify so the issue doesn’t cause damage to your property Hope this helps RYNOS RENOS

Sounds like condensation. ,