Councillors when elected, agree to follow the council's code of conduct which includes the 7 Nolan Principles of integrity, honesty, openness, selflessness, objectivity, accountability and leadership. If you feel that one of these principles was broken, then you need to report them to the Monitoring Officer. After that, sadly, very little will happen as there are no legal sanctions against councillor behaviour unless they have broken the law. I cannot see how an NDA could be legally enforceable in this circumstance.