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Like who is responsible for which side of the boundary features
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.
I don't remember seeing any manhole nearby or in the garden
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?
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I didn't have a manhole cover for my house at all (until I had my extension done when I had to put a new sewer run in and a rodding point in the garden). The manhole cover for the terrace I live in, was actually 3 houses away in the neighbours garden which served the entire terrace. It's not uncommon.
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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.
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