Re:
Have you checked the outside gutters?
Most people forget these and over the years fill up with crap/moss, leaves etc to a point that the water builds up and starts to flow into the walls and through them over time, Checking the gutters is a relatively easy thing to do with little or no cost.
Then try the loft, sometimes it can be a simple thing as one slipped or broken tile. Check those directly above the damp wall, it will be reasonably easy to spot if it is the case.
Have you checked the outside gutters?
Most people forget these and over the years fill up with crap/moss, leaves etc to a point that the water builds up and starts to flow into the walls and through them over time, Checking the gutters is a relatively easy thing to do with little or no cost.
Then try the loft, sometimes it can be a simple thing as one slipped or broken tile. Check those directly above the damp wall, it will be reasonably easy to spot if it is the case.