I dunno about bar stewards Bad One. Most farmers I know would never deliberately pollute their waters or streams. Its not in thier interests; most are very concerned about their land and water. And its often accidental or neglect. Farmers are very busy and checking every bit of equipment or valve gets missed.
Could the EA or the Angling trust do more about this. prevention is better than cure. Could they set up a scheme were farmers could log their equipment onto a computer program and when it needs checking or replacing they could be sent a email reminding them that this valve or this bit of equipment etc , needs replacing, over hauling etc. This could all be done on automation. It would take a bit of setting up but, not much cost involved and I think a lot of farmers/land owners would welcome it. After all it could save them a lot of money from being prosecuted after a pollution event. And I think, would prevent a lot of these slurry pollution cases happening. Its often down to neglected valves, old equipment etc.