Do I own the arch which supports my property?

I have a listed 18th century townhouse in the midlands. My adjoining neighbour enters his period property by vehicle through an arched entrance between both our houses. At the back of this entrance, the original arch beneath my period property remains where my neighbour has put a 4 inch sewer pipe through the springing point of my supporting arch. He got no part declaration and he did not ask me. Am I right to be concerned about this? I am concerned it would interfere with selling the property. Who owns the arch under a flying freehold?

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