For five long and dreadful years, at every opportunity possible, Kate and Gerry McCann have stressed one fact: they have never seen any evidence that their daughter Madeleine is dead, and so they hope fiercely and assume she still lives.

They voiced this belief again and again after the three-year-old vanished from their holiday apartment in the Algarve in May 2007. They insisted on it as Portuguese police suspicion fell on them ? unjustifiably, the country's attorney general later found ? and as parts of the British media libelously insinuated that the couple were involved in the child's disappearance.

They clung to their belief in the summer of 2008 when Portuguese investigators, lacking ideas and weary of criticism of a manhunt that had been bodged from the start, abandoned the search, leaving the two doctors to hunt alone for their daughter.

Horribly slandered but undeterred the couple gathered what resources they could and hired private detectives.

"We'll never give up on finding her, how could we?" Kate said last year. "What parent would give up on their child?"

On Wednesday that dogged persistence was rewarded when a team from the Metropolitan police, who have been reviewing the initial investigation after an intervention last year by the prime minister, agreed with the McCanns.

Chief Inspector Andy Redwood said he "genuinely" believed Madeleine could still be alive.

Crucially for the couple, this assertion is based on more than simply an absence of information to the contrary. Though Redwood would not be drawn, the Met position is based on new leads uncovered after a trawl through overlooked evidence, data that had not been analysed, and disregarded clues that the swamped Portuguese investigation was not considering.

This is not a new, hopeful, sighting of a pretty blonde child in Spain, Morocco or Australia; there have been dozens of those and all came to nothing.

Redwood called this "genuine new information". It could prove to be the smallest of leads, and Portuguese officers have yet to agree to the Met request for the case to be re-opened. But it is something.

If the announcement has offered an important vindication for the couple, it also represents an opportunity for British policing to redeem itself with regard to this situation.

The early decision by Leicestershire police ? the "home force" of the McCanns, who live in Rothley ? to stand back in favour of Portuguese investigators was perhaps understandable given international protocols. But by the late summer of 2007 Leicestershire was closely involved in the investigation, lending specialist sniffer dogs and forensics experts to the hunt.

It was, the attorney general found, largely due to a catastrophic misinterpretation of the evidence collected by these officers that the Portuguese team came to suspect the McCanns in the disappearance. A blinkered investigation, prejudicial police leaks and a rash of misjudged headlines followed.

Last month, Matt Baggott, at the time chief constable of Leicestershire, admitted to the Leveson inquiry that he had known the Portuguese officers, then heavily briefing reporters that the McCanns were guilty, were wrong on crucial DNA evidence.

He could have corrected reporters' errors, even behind the scenes, he admitted, but had judged it better not to.

The damage done to the hunt for Madeleine by the tarnishing of the McCanns might never be known.

The couple did not attend the police briefingon Wednesday .

After four bruising years, during which they received almost no police guidance and were forced to rely on PR experts for help in negotiating both the enormous media interest and the investigation, they were  last year assigned a Met police family liaison officer. This officer keeps them up to date with developments, out  of the public eye.