Ongoing water pressure problem

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I have a problem with the tenants of the maisonette below mine. I have been told by a plumber that the reason we are not getting good water pressure from a shared supply is over use of appliances/showers from downstairs. There may be too many people living there. My question is how can I enforce the owner to pay towards me getting a seperate water supply? The freeholder says that he is liable to pay his share according to our leases. Another problem is I haven't got an address for him only a mobile no.