How much do police spend when they launch a cold case review? Inevitably, a lot.

No reinvestigation is cheap, but the costs can vary enormously depending on what needs to be done.

It comes as the high-profile unsolved case of Suzy Lamplugh, who vanished 40 years ago today, is set to be mothballed next year.

The estate agent's disappearance sparked one of the largest ever missing persons investigations in the UK.

But now the Metropolitan Police is set to move the inquiry to an "inactive phase" in 12 months' time, meaning that while new information will be followed up, the team will no longer actively work on the case.

The decision to reopen an unsolved case will hinge on its solvability, though many forces regularly reconsider failed investigations.

It could be an initial re-reading of the case files to look for missed leads or new forensic opportunities, though retrieving boxes of archive material and loading it into a computer is time-consuming and clocks up staff costs even before the detective work begins.

Or it can be a specific search based on new information or a full-blown re-run of an old murder inquiry.

In 2024, acting on information supplied by the victim's family, the Metropolitan Police spent £160,000 on their eight-day fruitless excavation at a Hertfordshire farm in the search for the remains of Muriel McKay, who was kidnapped and murdered in 1969.

They had carried out a similar dig two years earlier, again on details from Mrs McKay's relatives who had tracked down and interviewed her surviving kidnapper.

Since 2011, the same force has spent more than £13m – funds provided by the Home Office – on its investigation into the disappearance in Portugal of toddler Madeleine McCann.

It's probably the UK's most expensive cold case review, which began after Madeleine's family urged then prime minister David Cameron to help, and it continues, though the Scotland Yard squad is much reduced and German police now lead the reinvestigation.

In 2015, West Yorkshire Police were allocated £1.5m from their police and crime commissioner to go back and revisit 55 unsolved murders, including one from 1974. The funds helped them hire 35 temporary staff.

The biggest financial burden of such investigations is manpower, though new forensic and especially DNA tests – often the key to solving an old case – can add hugely to the budget. The re-testing of delicate decades-old clothing, for instance, can cost thousands of pounds per item.

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To limit spending, there is increasing collaboration between the 43 regional forces in England and Wales, such as the East Midlands Review Unit, which covers Derbyshire, Leicestershire, Lincolnshire, Northamptonshire, and Nottinghamshire. The shared unit was said to save the forces an annual £300,000 in overheads.

Who knows what the Suzy Lamplugh investigation has cost the Met Police since 1986, but over the years, following tip-offs, there have been several excavations in the search for her body.

And there have been reviews, one of which identified crucial mistakes that could have led to detectives charging the main suspect many years before he died.

Three years ago, new forensic tests were begun on a swab of a smudged fingerprint retrieved from the rear-view mirror of Suzy's abandoned car. This month the Met revealed it still hadn't been able to identify the print.

It said the investigation into Suzy's disappearance and murder would continue for another year while they tried to trace 30 witnesses who may still have vital information. After that, without a breakthrough, the case would be put into the "inactive" file.

In the UK, unsolved murder cases are never closed, but they become dormant until something provides a good reason to look again.

This year, two brothers were convicted of the murder of civil servant Anthony Littler in London in 1984, after their younger brother came forward and told police they had boasted of the killing.

And last year, Ryan Headley, 92, was found guilty of raping and murdering Louisa Dunne at her Bristol home in 1967, perhaps the oldest cold case to reach trial.

Headley was convicted after semen from Mrs Dunne's skirt and intimate swabs were submitted for DNA tests that were not available for another 20 years after the crime.

A palm print taken from a window of her home also matched Headley, who had since been jailed for two other rapes, once the DNA identified him.

Such successes clearly breed confidence; a cold case squad covering Bedfordshire, Cambridgeshire and Hertfordshire police forces has launched an ambitious project to review 5,400 unsolved sex crimes from the past 50 years.