Before the works
An overflow that spills while its own treatment works stays shut is not being overwhelmed by rain — the problem is upstream of the works, and local.
If the sewer network were genuinely overwhelmed by rainfall, the treatment works' own storm overflow would be discharging too.
When an upstream overflow spills on a day its works did not, the works still had capacity — so the discharge points to a blockage, a failed pump, or infiltration on that branch, not to a works too small for its catchment. That changes who pays and how fast: a branch fault is maintenance, chargeable now, not a capital scheme waiting for the next five-year price review.
What this needs to be reliable
The comparison needs both monitors working. Where a works' own overflow monitor was offline we record no result rather than a pre-works spill, so this is a floor. A treatment works itself is excluded — it is the end of the line and cannot spill before the works. Two works in the catchment have no monitored overflow at all, so their upstream overflows are listed as not assessed, not as clean.