Also worth checking whether boiler pressure (if combi) or header tank level dropping, which would indicate a leak in the heating, and if you’ve got a water meter see if it goes up during a period when nothing’s turned on
The damp to the wall behind the settee is showing a classic condensation pattern. Also, rising dampness would tend to leave a brown/yellow/white mark around the perimeter of the dampness.
However, that's not to say that there is some dampness within the under-stair cupboard, but excessive or abnormal dampness would be affecting the stair timber and skirtings and you would notice it. It may well be a cold spot for any other moisture that you are producing around the house - which would also cause that staining behind the settee.
If there was a problem there previously, then you would expect there to be existing mould or signs of previous mould or recent redecoration to hide it. If not, then it may well be related to the change of use and different use of the property
Take the lino up, wipe all the mould with a fungicidal wash and put your dehumidifier in there for a week or so, with the door closed. But you need to consider if you are producing excess humidity in the way you are using the house.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local