I can now see the damp very clearly - pics dont properly resolve themselves on my PC until a couple of days have passed.
Airbricks imply a suspended floor.
Given that you have two suspended floors - so, as i advised above, it would be best for someone to crawl under both floors and inspect the damp wall, & any joists or timbers near or in the wall.
If the oversite ground level is at different heights then maybe any DPC in the wall is being bridged?
One possibility is to open the wall at the damp patch and check the cavity.
Another possibility is to simply hack off the damp plaster and render back up in a sand and lime mix.
I know that you have examined the elec cupboard but perhaps return to it with a flashlight and inspect the wall again - look for black spots or discoloring.
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