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I still think that you are entitled to recover the water costs from the upstairs flats .
This should actually have been dealt with when you bought the property
because you would have been in a much stronger position, however , it was not and so it must be dealt with now.
Given the landlord knew there was a joint supply then surely he should be liable for the water bill and proportioning it based on usage. I feel as if I've been duped / misled given he was aware (and I wasn't). Also why should I still be expected to pay for the water supply for the entite building when I have no agreement from anyone that I can recoup any proportion of the bills?
Agreed, but by the same token you have no agreement to supply them either!
You are happy to supply I imagine if they pay. Their choice.
The only issue is whether LL pays or tenant. Usually tenant is responsible for bills.
I would write to tenants and tell them if they dont agree, take it up with LL. They know they havent paid for water.
Meanwhile pursue them for back money in proportion.
If you have lock up shop it seems likely that they use more water than you
I'll take it with the landlord (again) and get back to you hopefully next week.
Whoever pays, you cannot be made liable for the water for the flats.