I wonder whether this happened upstream or downstream of the Sewage Works?

EDIT: It seems the incident was caused by anaerobic digestate  getting into the River Mole a few kilometres upstream of the sewage works.  It appears (according to unconfirmed reports) that heavy rain caused a tank containing the digestate to overflow and this, allied with low water levels in the river, killed the fish. If the water levels had been higher the spill would have been far more diluted and thus caused less harm. 

We are investigating a pollution incident on the River Mole, near South Molton, after a member of the public reported seeing dead fish. At the moment we are not certain of the pollutant or the source. We have a team on site taking samples and gathering evidence. pic.twitter.com/qcMMtfdSZy

— Environment AgencySW (@EnvAgencySW) July 31, 2019