The line suggests the absolute certainty with which the accused Adams was perceived to be guilty in the exposition of the trial. The line that precedes it helps to establish its significance:
This was not one of those cases of circumstantial evidence in which you feel the jurymen's anxiety because mistakes have been made - like domes of silence muting the court.
There is a certainty in this particular case. The narrator makes it clear that the collection of eyewitness testimony which will corroborate the defendant's guilt. The certainty was evident "when the Crown counsel outlined his case believed that the man in the dock stood any chance at all." In this context, there was a certainty in the guilt of the accused. The line helps to establish the exposition of the story, one in which there is a sense of knowing and certainty in the trial's outcome. This will be challenged in the events of the trial and what will follow when the trial ends.
The line appears in the 2nd paragraph of the story.
The meaning is-
the jury is afraid of making mistakes because the court loses its trust with the people when it makes mistakes.If this happens,the court is silenced.but,in the peckham murder case,no such situation would arise because their were four eye-witnesses to testify against the accused murderer Adams and hence it is not possible that they are mistaken.
Hope it helps you.