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Leak running along the side of an internal wall - please help!

Every time there is heavy rain there is a leak running along an internal wall of our tenant's café. The building is a two storey from around the 1950s and has concrete exterior cladding. The café is on the ground floor and there are two flats above it. We've established that it's not coming from the plumbing upstairs but we don't know if the leak is coming from the roof or if it's coming through the side wall or windows.