Four years ago, construction workers clearing the land for a new hospital in northern France made a grim discovery.

Soon after, they had unearthed the remains of more than 100 people.

Such finds are not unusual in this part of France and the police knew exactly who to call.

Stephan Naji, the head of the recovery unit from the Commonwealth War Graves Commission (CWGC), arrived on the scene within hours.

It was the start of a painstaking investigation which eventually solved a 100-year-old mystery by identifying two missing Scottish soldiers.

Stephan runs a team of specialist archaeologists who are on standby 24/7.

Once they confirm that soldiers’ bodies have been found they remove the remains and any artefacts and extract as much information as possible.

The building site at Lens had been on the Western Front during World War One and was the scene of the Battle of Loos, one of the biggest in the conflict.

At the commission’s headquarters near Arras, Stephan showed us trays of objects which had been found there, including parts of boots, rusted belt buckles, shoulder badges, buttons and regimental insignia.

Stephan’s team analysed everything that was found.

Thousands of soldiers are still missing in action in the area, and the discovery of items from individual regiments enabled them to narrow down who the individuals might be.

Shoulder badges indicated that the remains could include soldiers who had fought for two Scottish regiments – the Gordon Highlanders and the Cameron Highlanders.

But to establish who they were it was time to pass the case on to another specialist unit based in the UK.

A nondescript office block at the far end of the Imjin barracks in Gloucester is home to a small Ministry of Defence unit called the Joint Casualty and Compassionate Centre (JCCC).

The team is tasked with identifying British troops who were killed in historical campaigns.

Not many know about their work, but those who do have nicknamed them the war detectives.

Nicola Nash has been a case worker at the JCCC for 10 years.

In 2023, she was told about the remains which had been found in France two years earlier.

After looking at the list of the Cameron Highlander soldiers who were still missing from the Battle of Loos, Nicola narrowed the search to individual names which could be checked on the 1911 census.

Nicola also noticed an unusual detail. One of the men had been found with some tiny buttons from the Newcastle Corporation Tramways.

“That was obviously quite unusual for a Scottish soldier. I wondered if I could use those buttons to try and make an identification,” she said.

Among the handwritten names which fill the columns of the 1911 census, she found a tobacconist assistant called Gordon McPherson.

“The key information was that it’s got James, his father, and he was working as a storekeeper for the Newcastle Corporation Tramways,” said Nicola.

The team needed to be certain that the remains were those of Gordon McPherson. That meant they had to track down living relatives willing to carry out a DNA test.

“When I’ve got the name of someone who is probably alive, I will then start things like Googling them. Are there any news articles about them? Do they have a LinkedIn, can I find them on Facebook?

“It’s through this that I am able to eventually trace them,” Nicola said.

This search led to two brothers, Andrew and Alistair McPherson, from Whitley Bay in North Tyneside.

When they were boys, they had been fascinated by what they called “the black box” – a family heirloom which has been passed down the generations.

On visits to their grandparents, they would ask for the box to be brought down from the attic so they could rummage through the contents.

Inside there was a musket ball from the Boer War, medals, citations, and letters – some of which are now barely legible.

Now, with both men in their 60s, it is the heart-wrenching letters written by their great grandmother searching for her lost son that bring tears to their eyes.

Regimental war diaries had shown that their great uncle, Gordon McPherson, had been killed on the first day of the Battle of Loos.

But his body had never been found.

Then, last year, Alistair received a letter out of the blue from the Ministry of Defence.

It said remains had been found in France which could belong to his relative.

“I was shaking like a leaf,” Alistair recalls.

The letter, from Nicola Nash, said it was believed the body could be Gordon’s because two buttons from the Newcastle Corporation Tramways had been discovered with the remains.

“My great-grandfather was the chief inspector of Newcastle Tramway, so our assumption is that he’d given Gordon the buttons for luck – and it was lucky because it helped identify his remains,” Alistair said.

A DNA test was carried out which eventually confirmed a positive match.

The investigation had established that they had found the body of Gordon McPherson, a 23-year-old Lance Corporal of the 7th Battalion of the Cameron Highlanders.

He had lived and worked in Edinburgh as a tobacconist before the war.

Gordon and his brothers Jim and Charles served in the Army during WW1. Charles, a bugler, was aged just 14. Their father James McPherson, the Regimental Sergeant Major of the Northumberland Fusiliers.

Andrew McPherson said there were tears when it was confirmed Gordon’s remains had been traced.

“The story of him has always been really present in the family and everyone’s always wondered what happened to Gordon.

“It feels like an absolute miracle that he’s been found.”

However, the identity of the second Scottish soldier was proving more difficult to establish.

It was thought he had been serving with the Gordon Highlanders, and buttons found on the site in France hinted he could be an officer.

But the search was complicated because he was discovered along with five different sets of remains.

Using war records, Nicola Nash discovered that 14 officers from the regiment had been in the area.

She traced the families of every officer, then DNA tests were carried out.

This eventually found a match with the family of James Grant Allan, a 20-year-old lieutenant.

It turned out that his great-nephew lived just three doors away from another member of the war detectives team.

The Allan family hail from Scotland but were brought up in Stroud in Gloucestershire, where Nicholas Allan has run a cafe for more than 20 years.

He and his siblings grew up knowing all about the part one of their relatives played in the Great War.

Nicholas had been at work when he got a phone call from Nicola Nash telling him his DNA sample was a match and that remains found in France were those of his great uncle James Allan.

“It stopped me in my tracks,” he recalled. “The hairs on the back of my neck went up. I was like: ‘Oh my God, why is this happening? Why particularly him [James Allan] and not a thousand others?'”

Only the night before, Nicholas had discovered a family album of photos and letters, many from his Great Uncle Jim.

Nicholas’s younger brother Christopher and his sister Rebekah have been re-reading the letters which were sent home from the front line.

“He was just a little boy… I feel sad that he didn’t live his life,” said Christopher.

Nicola Nash said she felt “a really special connection” with the two soldiers because of how much she learned about their stories.

“And I feel like I have a special connection with the families as well, because they have both been so involved and so brilliant,” she said.

This week, Gordon McPherson and James Allan were laid to rest at the Loos British Cemetery – just a few hundred yards from where they were found.

Alistair McPherson said he was delighted that his relative had finally been found and laid to rest.

“I’m just really, really emotional in a good way, a positive way,” he said. “We couldn’t have wished for a better send-off after all this time.”

After the ceremony, Alistair and Andrew were presented with the Newcastle Corporation Tramways buttons in a small framed case, and a folded union flag.

“We’re going to need a bigger black box,” added Alastair.

Nicholas Allan said he was “in awe” of what the war detectives had done, and that he was “so grateful” for their work.

“It’s just been so heartwarming and a real privilege,” he said.

In the past 10 years the CWGC recovery unit and the war detectives have found and buried the remains of more than 300 British soldiers.

While the majority could not be traced, their work has identified 60 of those who died in battle.

Many thousands more are still missing. Some will never be found – but the war detectives say their work will never stop.