A viable case of manslaughter means it was open on the evidence led at trial for the jury to conclude that the appellant was not guilty of murder but was guilty of manslaughter. Where manslaughter should have been left but was not, the proviso cannot save the conviction unless the appellate court is persuaded beyond reasonable doubt of guilt of murder on the record — and even then, the nature of the error (loss of a real chance of a manslaughter verdict) may itself constitute a substantial miscarriage of justice. The Crown Prosecutor has an obligation to draw the trial judge's attention to the need to consider leaving manslaughter as an alternative verdict.
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