Water leak from upstairs

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Hello,
I own one rental in a house consisting of 4 flats, all privately owned. The two downstairs flats have a running issue with the two uninterested owners of the upstairs flats. I have water stains appearing on the ceiling, a dripping overflow pipe is causing damp in a wall and about twice a year water gushes, not leaks, but gushes from a light fitting in the ceiling. I need to gain access urgently to both the flats upstairs to establish exactly where the leak is. I get little or no response from the two owners of the flats.
I will be speaking with my solicitor on Monday, but has anyone else had problems with unresponsive and uncaring co-owners who seem happy to let their investment deteriorate and frankly put the lives of their tenants at risk. Hugo