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The land registry entry should detail the responsibility of each boundary. All ours always have.
As for extensions over drains if the extension is old it may date from a time when the local council was responsible for such things and no permission could be sought or given (other than the generic planning permission). Don't contact the current water company or they will turn you down and insist the extension is removed. Liability insurance and move on.
If they have written something like "left hand wall belongs to us, right hand wall belongs to neighbours" there is a high chance that it's received wisdom bollocks.
It used to be common to put these under kitchen islands with covers you could lift, but memory tells me that's against regs now?