Who stole Scotland's first ever coins?

"You could put my name in your paper," he says, all friendly, all jovial. "But I would shoot you."

I've just rung his bell to accuse him of a crime. But he isn't in. So his partner, albeit sulkily, has passed me a Swarovski-encrusted iPhone.

The ex-con – a senior member of an underworld family – knows exactly what to say. "It wasn't me," he responds with another laugh. "It couldn't have been. I was in the jail."

His door has been knocked before. Police have been here. So have undercover private detectives. Now me. We are all looking for the same thing: hundreds of small, shabby pieces of metal, maybe 2000 of them and none much bigger than a thumbnail.

This is the Stewartby collection, the most important hoard of 12th- and 13th-century silver pennies, the first money minted in Scotland, ever assembled. It is, according to one expert, "our national treasure", the key to the economic history of the then still young Scottish kingdom. The collection's owner, historian and numismatist Lord Stewartby, was going to give it to the nation.

The coins were stolen during in a break-in at Stewartby's home in the Peeblesshire village of Broughton almost exactly five years ago on the night of June 7, 2007. The case has been rumbling on ever since, barely noticed.

Yet the story has been played out on a prominent stage: the house that was robbed, Broughton Green, was a family home of John Buchan, a pioneer of British thriller writing and grandfather of Lord Stewartby's wife Dorothy. And the story has quite a cast: underworld movers and shakers, shady coin dealers, a peer, private investigators and a dogged policeman.

Some of those looking at the crime suspect the man I am speaking to may have taken the collection. Or, at least, that he may know what happened to it. If so, the news is not good. The fact is there is no evidence.

"It will be in a field somewhere," explains the ex-con, once regarded as among the most prolific and expert housebreakers in Scotland. "They keep saying the coins were stolen to order. That's rubbish. Some boy went in to that house and got the stuff and then found out it was historic so he has chucked it. Too risky.

"Me? What would I do with it? If it was me, I would give it back. Because it is not like you would buy that kind of stuff. That's our history. I've the polis at the door and art recovery guys too and I have told them the same thing."

Later I talk to a law enforcement source, explaining that the suspect had suggested the coins were in a field somewhere. "Did he say which field we should search?" said the source with a chuckle. "Because we have no idea at all."

Everybody – crooks, cops and coin collectors – uses the same word for the crime: a mystery. The police don't have a firm theory – despite using the kind of resources and know-how, including the latest in DNA technology, they would normally devote to a whodunnit murder.

Could the ex-con be right? Were the coins taken by, say, desperate drug addicts who threw them away? Or could they have been stolen to order for some mysterious collector? Was it an inside job? Or a chance housebreaking?

The Stewartbys have been thinking about this. A lot. They don't think it was an inside job, that someone close to them betrayed them. "I think it must have been random," says Lord Stewartby slowly, as his wife distributes "the good biscuits" in Broughton Green. "Yes," Lady Stewartby echoes him. "We think it must have been random."

Now 77, Lord Stewartby, once a minister in Margaret Thatcher's government, had planned to spend his retirement sorting and cataloguing a collection he had built up over more than 60 years. Why? Because he wanted to leave something to the nation. Lord Stewartby had been storing the coins in the loft of Broughton Green as he took them from his London home to the National Museum in Edinburgh to be photographed.

Nobody has done more for Scottish numismatics than Lord Stewartby. In his youth, he literally wrote the book on the subject. The Scottish Coinage was published in 1955 by the then Ian Halley Stewart, age 20. It is still the authoritative account of the money issued by independent Scotland. Lord Stewartby researched and wrote much of it before he left school.

He started collecting his coins when he was six. I ask him what his first coin was. He instantly cheers up. "It was 1941," he explains. "I was in a shop and a grocer had a jam jar full of old coins on his counter. Like a lot of shopkeepers he was collecting them to be melted down to make munitions.

"I spotted one with two heads. Now, I had just been given this chart of all the kings and queens of England. So when I saw the two heads I knew it must be a William and Mary because that was the only time there were two monarchs ruling jointly. The grocer said I could keep it. It all sprang from that, a William 'n' Mary halfpenny."

Lord Stewartby still has that coin. But he has lost his most precious part of his collection: up to 2000 of them from the reign of David I to Alexander III.

The coin collection won't match the public perception, he warns. It isn't a chest of gold doubloons and pieces of eight, but a number of long, thin boxes full of little white annotated envelopes. And in each envelope is a small silver penny.

"They don't look like much," he says. "Like dirty little 20 pence pieces." Some of the boxes are from After Eight chocolate mints. "Don't say that," Lady Stewartby jokes. "Do you want to be thought of as a toff?"

These little pieces of metal, however, could not be more important. While each coin is worth relatively little – although rare, they are regularly dug up out of old middens by metal detector enthusiasts – put together they represent something bigger than the sum of their parts. This is the fullest picture of the first money minted by the monarchs of the House of Dunkeld, starting with David I. More comprehensive than that of the National Museum of Scotland, the collection could tell historians how much money was circulating in medieval Scotland; who we traded with, where our centres of commerce were. And because Scotland has no surviving portraits of Dunkeld monarchs, such coins offer the only (albeit inaccurate) images of what our kings looked like.

Their value? The police have said £500,000. But in reality the collection is both priceless and nearly worthless. Carefully catalogued in a museum it is worth a fortune. Melted down as cheap silver it is worth next to nothing.

Yet the collection does have a real, practical value on the criminal market: £50,000. This is the reward put up by Lord Stewartby for information leading to the coins' safe return. And yet nobody is biting.

The couple know the coins aren't on the open market, because the tight-knit world of coin collectors would spot Stewartby coins if they appeared for sale. Any coin that could potentially be Lord Stewartby's is being DNA-checked. None, so far, matches. So why is nobody coming forward to collect on the reward?

"We don't know what the blockage is," says Lord Stewartby. "It may just be that the people who have got them have not known what to do with them. It could be all sorts of things. It could be a dispute between the finders on how you would split up a reward. I just hope they know the money is there to be claimed."

This – say insiders – is how some thefts of art and antiques work: you steal something then try to sell it back to the person you stole it from. Because, they say, fencing artworks or heritage items isn't easy. Typically such goods command just a 20th or a 10th of their commercial value on the black market. Provenance, after all, is everything. Without it, prices tumble.

There are an estimated 350,000 stolen artworks floating around the world, according to the FBI and Art Loss Register. The top eight or so stolen paintings would, collectively, be worth perhaps a $1 billion at auction. Even 5% of $1bn is a fortune. So there is a tidy little underworld market for such goods, and a growing industry of private investigators and art recovery experts whose job it is to rescue them.

James McKenzie is such an expert. A Scot and former police detective who works for international security consultants Red24, McKenzie is the man leading the effort to get the Stewartbys' coins back.

And it is a huge effort. McKenzie is old school, a believer of the ABC school of policing. "A is assume nothing. B is believe nobody and C is confirm everything," he says. His firm, he adds, is doing "everything that is lawfully audacious to find out what has happened to Lord Stewartby's coins".

Lawfully audacious: this is the expression police use when they are talking about cunning tactics, such as telling insurance companies the names of men they believe to be gangsters – and driving up their premiums. For McKenzie – and this is just one of several names he uses – it means investigating a crime with the resources of a multinational company but staying on the right side of the law.

Speaking by phone from his London HQ, he says: "So far in this investigation we have deployed a surveillance team and undercover officers. Such officers have met with criminals, with suspects whose names have cropped up. They have visited known handlers, someone who has been convicted of handling stolen goods, or someone who has handled stolen goods but has never been caught.

"We are a global company. I can deploy someone in any country within 24 hours to bottom out any information we have got. This is something not many companies can do. The financial situation within the police service is dire. And the resources they have are focused on things like paedophiles and murderers, which must take priority.

"We haven't got those problems. But a day doesn't go by when we don't think about the Stewartby case. Because Lord Stewartby was not collecting these coins for personal gain. He was doing it so it could be in a museum in Scotland for the benefit of our kids and their kids."

McKenzie admits his team, like the police, still don't know what happened to the coins, but he has some ideas. "Maybe someone is just hanging on to them for as long as they can and then hopefully get rid of them in years to come when everything is calmed down," he speculates. "Or maybe they are releasing them on to the market, one or two at a time. Because that is hard to detect.

"Or they could be dumped. But I feel anyone dumping them would have gone back to collect them. You are not going to risk jail and not try to find out what you have got. Or it could have been a couple of idiots who stole the coins and now they are lying under a bed in a Glasgow tower block.

"Nowadays art can be used by organised criminals as a commodity, as collateral, in deals. It could be swapped for drugs or sold to finance crime. Some idiot may have melted them down or sold them to a scrap metal dealer.

"It is not high-value metal. They would not get much for them. If the thieves are drug users they might sell them for £50, not £50,000."

McKenzie, like Lord Stewartby, wonders why the reward hasn't been claimed. Could whoever stole it not forward the collection on to someone else, at a discount from the value of the full reward?

His gut instinct is that the Leonardo case, Scotland's other big antique theft of this century, may have frightened off some would-be reward claimers. Two years ago a group of five men – including two Glasgow solicitors – were cleared of conspiring to extort £4.25m for the safe return of Leonardo Da Vinci's Madonna Of The Yarnwinder. It had been stolen from Drumlanrig Castle near Dumfries in 2003. One of the men later told reporters he believed he was entitled to a finder's fee for the painting.

"There have been certain court cases," McKenzie explains, "that may have put people off coming forward. It could be people have never done this type of crime before so they are scared in case they come forward and get arrested."

But Red24 isn't interested in prosecutions. They just want the painting back. "As a company, we would be looking to recover the items and people that give us the information would receive that reward," McKenzie says. "Anybody with information would not be dealing with the police, they would be dealing with us. We are still pursuing certain lines of inquiry. There is no case that is closed. They will always remain open. We think about it every day."

McKenzie's job is to recover the coins. Stevie Hall's job is too. But the Lothian and Borders detective – who has been investigating the Broughton raid from the get-go – also wants to catch the culprits. Hall stresses that the police, like Red24 and the Stewartbys, don't have a firm theory for the crime. This is despite his team using the kind of intelligence and resources they would customarily devote to a murder.

Who does he think took the coins? Hall – for whom the Stewartby case is the longest of his career – shrugs. He's not ruling out the idea of a lone collector ordering the collection from Scottish bandits. But he doesn't sound convinced this is likely. "Well, it seems there are a few collectors in America who are always looking for Scottish coins," he says, "but if this was a theft to order that means whoever got them would have to sit in a cellar enjoying them on their own. They wouldn't be able to share them, they wouldn't be able to show them off to fellow collectors. And they wouldn't be able to sell them at anything like their real value. Because the coin world – which is a pretty small community – would be confident that they would spot coins from Lord Stewartby's collection appearing on the market."

The housebreakers could have just got lucky and found the coins, he stresses. But he does think the break-in was done by people who were far from novices. "There was no significant damage done to the house and I would be confident in saying the thieves were professional and organised."

The thieves came back. The same ones? Maybe not. But there was another break-in at Broughton Green after the first. The building is now secured like no other in the Borders. Red24 have seen to that. But the Stewartbys have to live with the knowledge their home has been invaded twice.

Lady Stewartby likes a good story. After all, her grandfather, John Buchan, wrote plenty, including The Thirty-nine Steps, and she helps run a museum in his honour. Friends tell her the coin theft could have come straight out of a Buchan thriller.

She likes showing visitors the thick-walled part of her Broughton home that served as an inn in the 18th century. She points to where Jacobites stopped for a drink on their long march south in 1745. But her own story is wearing her and her husband down. They bought a terrier, Mabel, after the break-in, trying to fill the gap made by Lord Stewartby losing what he sees as his life's work.

"The crime itself, the burglary, just made me furious," Lady Stewartby says, tugging on her dog's fur as it sits on her lap. "I wasn't going to give the burglars the satisfaction of minding or contemplating moving or anything like that. But it has had an effect on us, even if we don't want to admit it. You lose your peace of mind. If you wake in the night, of course you are listening out for footsteps. A burglary ruins your peace of mind every time you leave the house. Do they know what a cruel thing they have done? I don't think they care."

Lady Stewartby has led the campaign to get the coins back, desperate to return them to her husband as the years tick by. Because she knows he is uniquely placed to catalogue his own collection. Scottish numismatics would be set back, one expert told me, by at least five years if the coins are returned and Lord Stewartby is not fit enough to sort them out.

His wife is giving Mabel a full body massage as she describes her fear that her husband won't get the collection back. "The worst effect is seeing somebody you love being so unhappy," she tells me through tears. "I am the sort of person who gets things sorted, whatever the problem – death, illness, you just get out and sort it. But I can't sort this because I don't know what I am looking for and I don't know where to start.

"And I don't want Ian to live out his last few years wanting something he hasn't got." n

Do you know where the Stewartby collection is? Ring David Leask on 0141 302 6637 or email david.leask @theherald.co.uk.