One thing that I think is important to acknowledge is that with many unsolved murder cold cases, the police do have a relatively good idea as to the identity of a perpetrator.
However, because the CPS act as a form of judge, jury and executioner, it is they who make the decision as to whether a case has enough evidence to go to trial.
Therefore, from the perspective of the police; it's not about what you know, it's about what you can prove.
And if the CPS feel that the police don't have a strong enough case to proceed to a potential conviction, they will shut it down as they see fit; often for the sake of promoting positive statistics. (they're not going to back a horse with only 3 legs)
This leaves many cases in limbo, whereby the police are as close to certain as they can be, but they are stopped in their tracks by the system that determines how the case progresses; if at all.
I am not suggesting that the case of Melanie Hall is one of those particular cases, but in a way, I get the sense that it very well could be.
But that would of course imply that the police have a pretty good idea of who murdered Melanie Hall.
All I would say is that judging by the syntax used by Melanie's parents, it would appear that they also have a fairly good idea as to who took their daughter's life.
Unfortunately, with many of these cold case reviews, it becomes a lacklustre exercise of just ticking boxes, and unless a full review is undertaken with considerably more time, money, resources, and man/woman power put in, then it's extremely unlikely that new evidence will come to light, which could then potentially give the police that extra piece of evidence that they need to try and convince the CPS to consider progressing the case further.