The distinction insurers draw is between a sudden, accidental event and a maintenance issue. A pipe crushed by a falling tree is a claim; a pipe that has silted up over twenty years is not.
Where a claim is possible, evidence matters enormously: CCTV footage, a dated defect report and photographs of any damage. Get the survey before the repair, not after.
Home emergency cover typically has a call-out limit and a list of approved contractors, so ring them before instructing anyone if you intend to use it.