Criminal Justice and Immigration Bill

I wish to ask a question that is not intended to be a trick question and may turn out to be a silly one. What are the circumstances in which a presumption may be rebutted? I can understand a requirement that particular action should be taken, but where one comes across a presumption that it shall, one expects to see an explanation as to how that presumption may be rebutted. Perhaps that can be answered at some later time, if not today.