"28. While reference has been made above to some of the rules which should be observed in a well-conducted trial to safeguard the fairness of the proceedings, it is not every departure from good practice which renders a trial unfair. Inevitably, in the course of a long trial, things are done or said which should not be done or said. Most occurrences of that kind do not undermine the integrity of the trial, particularly if they are isolated and particularly if, where appropriate, they are the subject of a clear judicial direction. It would emasculate the trial process, and undermine public confidence in the administration of criminal justice, if a standard of perfection were imposed that was incapable of attainment in practice. But the right of a criminal defendant to a fair trial is absolute. There will come a point when the departure from good practice is so gross, or so persistent, or so prejudicial, or so irremediable that an appellate court will have no choice but to condemn a trial as unfair and quash a conviction as unsafe, however strong the grounds for believing the defendant to be guilty. The right to a fair trial is one to be enjoyed by the guilty as well as the innocent, for a defendant is presumed to be innocent until proved to be otherwise in a fairly conducted trial. 29. The crucial issue in the present appeal is whether there were such departures from good practice in the course of the appellant's trial as to deny him the substance of a fair trial ..."