A HAWKER acquitted of beating up an off-duty policeman will have to be tried again because of a magistrate's ''improper behaviour'' during the trial. Allowing the Crown's High Court appeal, Deputy Judge Daniell held that trial Magistrate Li Wai-chi erred in disclosing his view on a prosecution witness before the Crown closed its case. His behaviour was improper and had deprived the Crown of a fair trial, said the judge, ordering a retrial of Ho Chi-yuen. Ho, 37, had denied assaulting off-duty policeman Wong Ping-sum last June, occasioning actual bodily harm. During the trial, while Mr Wong was giving evidence, the magistrate had told the prosecutor that he considered his evidence to be ''wholly unreliable''. On appeal yesterday, Grenville Cross QC, submitted the magistrate appeared to have formed his view of the Crown's case while evidence was still being called.