They bring a fresh set of eyes to make sure nothing has been missed.

And they also allow the evidence to be viewed in the light of what we know now and they give new impetus to the investigation.

Forensic science has moved on and re-examination of the original exhibits may well lead to new clues.

Renewed publicity can locate new witnesses and this is the most important point.

Loyalties will have changed. People unwilling to talk to police about their friends four years ago may well now choose to do so.

The suspect is very likely to have said something to someone close or to have done something to raise suspicions in that person's mind. This is why an unsolved case review is essential.