Shared supply, but the leak was on one branch
The property
A Regency house, one of a pair fed by a single old supply pipe from the pavement, which splits under next door's front garden.
The problem
Anglian Water wrote that the meter never came to rest. Both households assumed the shared pipe had failed and expected to split the repair.
How we found it
Closing both stop taps left the dial turning. The ground microphone was loudest near the split, and tracer gas fed into each branch in turn surfaced only along this house's branch.
The outcome
The leak sat a short way along that branch. We marked and photographed it, quoted the repair separately, and the report showed both households which pipe it was on.