I would expect the EA to act quickly when they are notified. A potential incident further up the Weaver was stopped by the EA not too long ago. A walker spotted something on a farm which they felt was an imminent danger to the river. The EA were there within an hour and the pollution prevented.
My 'Act quickly' was based on the restocking (Peter Jacobs 10:04) - which is indeed good news provided they knew what had happened. If they don't know what happened or don't have a good idea of what happened, why restock?