Lord Stewartby's coin collection was said by experts to be unique. The former Tory minister started it when he was just four years old and, more than 60 years later, he had amassed almost 2000 coins, dating back as far as 1136 and valued at more than £500,000.

They included a silver penny minted under the reign of Robert the Bruce and others struck under James I and II. In short, it was the most historically important collection in Britain. A leading numismatist, the 72-year-old peer had retired in May and, anticipating time to concentrate on research, had taken his collection home to Broughton Green, the house in the Borders where 39 Steps author John Buchan once lived, to be catalogued. But it seems he was not the only person attracted to rare coins. Between June 6 and 7, while he and his wife were on holiday, the house was broken into and the collection taken. "It was such a great shock," he said at the time.

The £50,000 reward he has put up for information leading to its safe return speaks volumes about his determination to get the collection back. That means a select band of individuals may be wondering if the phone will ring requesting their expertise. A group of former senior police officers - most of whom worked for the Metropolitan Police's art and antiques unit - loss adjustors and international data-base co-ordinators are the UK's art detectives.

For the most part they insist that criminals behind art thefts are not really any different from any other. They reject outright too, the myth of a Dr No-type figure sitting in his nuclear bunker surrounded by precious masterpieces and fine antiques.

But it's certainly big business. Internationally, an estimated 10,000 works - collectively worth billions of pounds - are taken from museums, private collections and country homes every year. These supplement the catalogue of the already missing, which runs to some 479 Picassos, 347 Miros, 290 Chagalls, 225 Dalis, 196 Durers, 190 Renoirs, 168 Rembrandts and 150 Warhols. Internationally, the most famous thefts include that of 13 works, including a Vermeer and a Rembrandt and collectively worth $300m, from the Isabella Gardner Museum in Boston.

Among Britain's most notorious thefts are that of Leonardo Da Vinci's masterpiece Madonna of the Yarnwinder, taken from the Duke of Buccleuch's home, Drumlanrig Castle, Dumfries and Galloway, in August 2003. The thieves simply bought tickets along with other tourists and then held up a guide before pulling the painting, worth £70m, off the wall and walking out the door with it.

Their tactics might seem crude but Richard Ellis, a former head of the Met's art and antiques unit, who now works as a security and recovery expert for Art Management, says they should not be underestimated. "They are professionals who will do their research very carefully and know exactly what they are looking for before they strike," he warns.

According to those in the know, art is stolen not usually to order but by someone who knows they can shift works quickly to a handler for cash. Profits often go into drugs or arms, or the painting used as collateral in a deal.

"Another trend that we're seeing now is that pieces are being sold through eBay," explains Ellis. "There is no way of regulating that and police will admit that they just don't have the resources to monitor it."

The Met art and antiques squad, the UK's largest (it receives 120 requests a year for assistance on average), only stretches to four officers. It's no wonder the police can't cope.

But Ellis sees another problem. "What makes this type of criminal so successful is that the police are under pressure to concentrate on crimes such as anti-social behaviour but you have to be operating in the upper stratosphere to draw the attention of the organised-crime agency. Consequently there is no-one looking at this high-value professional type of criminal."

In recent decades several attempts have been made to tackle the problem. In 1989, with art theft on the rise, Ellis joined the Met and started a new phase in detection. In the 1990s he worked on high- profile sting operations with fellow officers John Butler and Charley Hill, recovering a Vermeer in an Antwerp car park in 1993. A year later, Hill posed as a representative of the Getty Museum in New York. Offering a $5m reward, he bagged Munch's The Scream.

Gradually budgets dried up, ironically creating a gap in the market for Ellis and his contemporaries to work independently. Ellis now works for a variety of clients including the Egyptian government, loss adjustors and owners. He travels all over the world following leads and recently recovered paintings worth millions.

But he remains frustrated at the number of UK country houses that are easy targets, claiming that the recent coin theft is one of a series sweeping the north of England and the Borders.

Mark Dalrymple, director of Tyler & Co and a fine-art loss adjustor charged with trying to return the Madonna to its owner, understands his frustration but says the thefts will continue because of one main factor - money.

"People aren't stealing these for their homes," he says. "They may be bathing in the afterglow and prestige that comes with such a major theft in the eyes of their contemporaries. But, at the end of the day, it's about making money."

One former stolen-art handler and author of the Art Hostage blog, who left his criminal career behind almost a decade ago, concurs. He explains how it worked. "When people came to me after a job, they would get a percentage of trade price. For example, if something was worth £3m and I could get £50,000 for it, they'd get £10,000. I'd pay them there and then so it's relatively easy money for a couple of hours' work. And you're much less likely to get caught than you would be holding up an ATM machine."

The handler would then pass it on to contacts in the antiques trade, who he claims were happy to turn a blind eye and would offer information to the police in return for them occasionally doing the same thing.

Often works would end up at London's Bermondsey Market, where, due to the ancient marche ouvert (open market) principle, an owner who had bought in good faith between sunrise and sunset got good title. "That meant you had people out thieving on a Thursday night, shunting it straight to handlers and selling it for cash at 5am in the morning," Dalrymple sighs. "Even if the buyers subsequently found out it had been stolen they wouldn't have it taken off them. It was daft."

In response, he founded the Council for the Prevention of Art Theft (Copat) in the late 1980s, a forum for the art world, insurers and police to work together. It resulted in the abolition of the market ouvert principle, and, for a while at least, better co-operation from dealers.

It may have helped but art theft is still rife and tracing works is harder than ever according to Dalrymple, who claims that, due to new legislation about protecting informants, most of his criminal sources don't bother to give him the badly needed tip-offs that led to the concealed masterpieces. He still has means, one of the most unlikely being the placing of ads in the trade press, which in 2002 led to the recovery of a Goya, stolen the year before.

Another helpful tool is the London-based Art Loss Register, which has recovered £120m worth since it was established in 1991. For a 15% finders fee they will rake through auction catalogues and have a presence at every major art fair, checking for pieces registered as stolen.

"The strength of the data base lies in that registered items will never get removed and that we work closely with the police and the insurance industry worldwide," says organisation spokeswoman Maya Bernard.

But, according to former police officer Hill, there's no substitute for good old-fashioned legwork. The maverick detective who developed a formidable reputation for his undercover work with the Met in the 1990s, now works freelance, flying round the world in search of the "big ones". When we speak he is on his way to Spain to follow a lead on the Gardner thefts committed 17 years ago in Boston.

But not everyone is a fan of his methods. In 2002, while on the trail of Titian's Rest on the Flight into Egypt - stolen in 1995 from Longleat - he enlisted the help of known art criminal David Duddin. That led to an approach by another man who said he could help, driving him around west London until he discovered the painting in a plastic bag near a bus stop. The man was paid a £100,000 reward, and Duddin also got a fee. Hill says he was satisfied that neither was involved in the crime, allowing him to pay the reward without breaching laws preventing criminals benefiting from their acts. Others said that by paying out he was encouraging works to be held to ransom. He insists he knows his limits. "You have to be confident that you know the difference between right and wrong," he says. "My test is that everything I do has to be both legal and reasonable."

The other requirements are persistence, a love of the chase and, perhaps most importantly, a fascination with the works. He confesses he is more interested in recovering art than catching crooks. "It's the art that I'm really interested in," he says. "Its historical significance sometimes outweighs its financial value, like that coin collection for instance. That is really important to Scotland. I'd like to think I could get that back."