sewer ownership

Other

I have two maisonettes with their own downstairs front doors. Their sewers run together and have been blocking. There is no management company and the two properties have separate water bills, so I was hoping the water co would now own the sewer.
Can I force the water co to fix the sewer when it blocks? Last time the tenant called the water co but they passed the buck back to me after failing to rod the drain. I fixed the blockage the next day quite easily but fear the problem will recur.