If he is on a Council Plot, then speak to the council, they have a duty of care and also by him dumping stuff on your plot he is effectively creating a legal nuisance, and again the council have a duty to intervene. If he has dumped asbestos then this is a criminal offence and once again the council have a legal duty to act. you are on a private plot so to speak, your landlord also has a duty of care to protect you and their land from unwanted nuisance and illegal dumping. Unfortunately my response would be to have a friend have a quiet word with him.