It was a potential breach of this for the judge to direct the jury that in assessing the defendant's evidence they could take into account his interest in the verdict.
Other examples of possible deflection of the jury from their task mentioned here are comment on the defendant's failure to give evidence  and asking the defendant in cross-examination why the complainant is lying .
Deflection of the jury from its fundamental task is a miscarriage of justice. Whether deflection has occurred in a particular case must be assessed in the context of the whole of the summing up and the circumstances of the case. As it turned out here, there was no miscarriage, because the jury would not have understood the judge to be saying that the evidence of a defendant must be scrutinised more carefully than that of other witnesses.