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is water running over the gutters on the roof and running down the outside wall? perhaps check that your gutters are working well - they may need cleaning/unblocking.
what am I brushing off of the wall and how do I know exactly where to do that?
The white paint. Wire brush in between the faces of the brick for now this should allow at least some of the moisture back out. Its not allowing any moisture to evaporate out of the brick work. You really need to get this looked at. If that really is a suspended timer floor then you might start having major issues if that damp has been there for some time you might find the ends of the timber joists have started to rot. Do you have any way to access under the floor and check?
thanks I will look at this soon.
I don't have any way to look under the floor except taking the skirting board off, floorboards off etc
wire brush - do I need to go back to the cement/mortar?
I guess the paint will flake off.
have a look at it - does the wall feel wet after its stopped raining for a few hours? is there actual water behind the paint? is the paint starting to bubble up because the water behind it cant escape? You might have an issue that its not just the paint that is causing the issue, looks like some of that wall has been repointed. If the house is old and they have used a cement mortar instead of an lime mortar that will also trap the moisture in the brick work.
Are you 100% sure you dont have a water leak somewhere in the house? Next time you go out turn off all the taps in house so nothing can use water. Check the meter and see what the numbers are, then when you return say 3 hours later check them again, this should tell you if you potentially have a hidden leak somewhere in the house.