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Watermark on ceiling

I’ve noticed a watermark on the ceiling up against the outside wall. I live in a block of flats on the bottom floor, the building is made from concrete block. We can’t get into the ceiling and have a look. There’s an overflow pipe outside very close to where the stain is, so I’ve wondered if that’s leaking and water is getting in. I don’t know if I need a builder or a plumber, if someone could take a look and tell me, or fix whatever is going on, that would be great.