I understand NJ's arguments for the double jeopardy principle and his insistence that we ought not make exceptions to it even in the circumstances of the Stephen Lawrence case.
But new evidence - as in my DNA/Hanratty example - could reveal a miscarriage of justice - like a guilty man has got off with murder.
Do we have another go at the guy or not? Double jeopardy and NJ say no. This would be consistent with the famous Blackstone formulation from the 18th century: "it is better that ten guilty persons escape than that one innocent suffer". There's a real point here, but Blackstone's use of arithmetic reduces it to absurdity: you can ensure the protection of the innocent by acquitting all, or, more simply, prosecuting nobody, thereby serving the needs both of economy and of justice as defined by Blackstone.
But turning my example around, if an innocent Hanratty had been jailed for life rather than hanged, then subsequent DNA evidence proving he wasn't the rapist and murderer would have secured his release, yes?