Getting it right in future is better than rewriting the past

No one doubts that the failure of the police and the Crown Prosecution Service to secure convictions for the murders of Stephen Lawrence and Damilola Taylor is distressing. It must add immeasurably to the grief of the families to have it repeatedly suggested that "we know who did it", without the prospect of bringing the killers of their loved ones to justice.

No doubt, too, that the police are sincere in their frustration in some criminal cases at being unable to use evidence that turns up after the trials.

But when Sir John Stevens, the Commissioner of the Metropolitan Police, makes clear in his interview in The Independent today that he regards the Government's proposed legal changes as a chance to re-open those cases, he is aiming at the wrong target.

The lesson of the failure of the authorities to convict is not to keep trying until they get it "right", but to work harder to make sure that they always put a credible case before the courts the first time round. The police and the Crown prosecutors ought to ask themselves why, in both the Lawrence and Taylor cases, so much of their "best" evidence turned out to be inadmissible. In neither case was this a matter of legal "technicalities", or defence lawyers "playing games", but because of the operation of tried and tested rules of law designed to safeguard innocent but unlikeable characters from hunches and prejudices being held against them.

One of those rules is that against double jeopardy – that it is wrong to try the same person twice for the same offence. The reason is simple: that it is so much harder to maintain the presumption of innocence at a second trial.

It would be wrong, therefore, to abolish the double-jeopardy rule. But it would be doubly wrong to do so retrospectively. It is another fundamental, but unwritten, safeguard of British law that it cannot be changed to criminalise past actions or specific individuals after the event. The defendants in the Lawrence and Taylor cases were tried on the basis that, if acquitted, they could not be tried again. Distressing though it may be, Sir John should concentrate his efforts on avoiding similar mistakes in future, not on rewriting the past.