If it is not illegal that it should be.
Is it time to fight fire with fire?
I suggest the Alyn Waters Dung Spreading championships or maybe the local car cruising club all night all week sound off. Why make it comfortable for them?
If they like the field so much just barricade them in. It is the councils land so if they choose to build a moat there fair play to them. If they won’t change the law to stop this sort of behaviour then it will take drastic action from a brave council to stop it happening in the area again and again. (which it is)