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The evidence behind it.
Coverage and liability are two questions. The evidence-first approach keeps the claim notice, the clause, the parties involved and the open points together without mixing them.
Covered only if it was escaping tap water.
Policy § 4.1: damage caused by escaping tap water is covered. § 4.3: damage caused by backflow is covered only if the natural-hazards extension has been agreed.
On the file, three parties come into question. Nothing has been decided.