The Tonbridge heist

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Dumping the money was the smartest thing the robbers could have done. When police found bank notes worth up to £15m in the back of an abandoned white van on Friday, it was easy to imagine that the gang responsible for the Tonbridge heist had left the money after panicking or being disturbed. But as police made two arrests yesterday, and an emotional statement was read out on behalf of the family kidnapped in the raid, another theory emerged.

Three highly professional south London gangs are now the main suspects and, money-laundering experts believe, leaving some of the loot behind may be another example of the robbers' meticulous attention to detail.

If the notes discovered in the van had been newly printed, they would be very hard to get rid of. "It would be very foolish to try to spend them or deposit them in a bank account in this country," said Prem Sikka, professor of accounting at Essex University. "The serial numbers would make them easy to trace."

Far better to abandon them and carry off only used ones that had been in circulation already and would be untraceable - and which may have been worth up to £35m on their own.

The van was found in the car park of a hotel in Ashford, Kent, close to the Channel Tunnel terminal. This suggests the notes may now have been smuggled out of Britain, possibly just hours after the raid - and experts believe they will be laundered through a complex series of transactions across the world, from the Balkans to an offshore haven such as the Cayman Islands.

"It's not the robbery that separates the amateur from the professional," said the former armed robber-turned-author "Horace Silver" yesterday. "It's the way you deal with the money afterwards."

The biggest bank job in British history looked like a victimless crime at first. The money was being stored at the anonymous, high-security depot in Tonbridge on its way to and from shops and banks across London and Kent. The insurers of Securitas agreed to pay the Bank of England an initial £25m (with more to come, when the audit is complete), so the theft will cost the taxpayer nothing. Audacious bank robbers have a certain glamour. But not to Colin Dixon, the 51-year-old depot manager who was flagged down by a Volvo saloon with flashing blue lights on the A249 near Stockbury in Kent as he drove home that evening.

The police car that stopped him turned out to be fake, but Mr Dixon did not realise it until he was handcuffed, before being moved to a van or lorry and driven to a farm. Mr Dixon had been trained in how to deal with being kidnapped. Securitas helps its staff prepare for such an ordeal as part of security measures that include fingerprint recognition systems and weight sensors which prevent more than one person from entering a room at the depot at a time. But even the toughest and most prepared security professional would have been shocked by the sight of his wife and nine-year-old son being brought in at gunpoint.

Lynn Dixon, 45, had answered the door at their home in Herne Bay to two men posing as police officers, who told her Mr Dixon had been in an accident. She agreed to get into their "patrol car" with her son, but the pair were taken in a red van to the farm where Mr Dixon was being held. The whole family was then moved to the depot, in the convoy of robber vehicles.

"This horrific experience angers me beyond belief," said Mr Dixon in his statement yesterday. "To kidnap my wife and child and threaten them with death is something so frightening that words cannot convey them today."

Who would do such a thing? The police thought they knew the answer straight away, according to sources close to the investigation. Finding evidence will be harder. The main suspects are three south London gangs, all led by families that grew up in the postwar poverty of the Old Kent Road and the failing docks nearby. These conditions have produced the most ambitious and dangerous armed robbers in British history. South Londoners were responsible for the £26m Brinks Mat bullion robbery of two decades ago, and the audacious but doomed attempt to steal diamonds from the Millennium Dome in 2000. Their dubiously acquired wealth has enabled many villains to move out of the city, mostly to Kent. This robbery was therefore in their home territory. The three crime syndicates have long been watched by detectives - the six brothers who make up the senior members of a gang nicknamed the A Team, for example, were under 24-hour surveillance for a whole year in the 1980s. Detectives will now be checking their records from the past few months.

In addition, the security companies that handle huge sums of money employ their own consultants, usually former Scotland Yard detectives. As one senior security industry manager said: "There are about 100 security consultants and 100 top-league robbers who spend their time shadowing one another."

So what happens now? If they have not already fled the country, then the first and most important thing for the gang to do is sit tight. Wait. Keep the money hidden for as long as humanly possible: in the ground, under tarpaulin, anywhere.

"After a big bit of work, everyone lies low," said Horace Silver. "You know the Flying Squad only has a limited amount of time to work each case with the full level of resources. After six or eight weeks, if they haven't made any progress, things start to get quieter. They've got 100 police on the job at the moment, but they won't always be there."

People on the fringes of the gang - a van driver or an insider at the security company - may be fed small amounts of money as a test. "You give them a little bit at a time and watch what they do with it. You have to warn them not to be too flash or start changing they way they live. When people suspect an inside job, the staff are the first people they start looking at."

The cash presents a physical problem. The IRA raided the Northern Bank in Belfast in December 2004 and stole £26.5m, but the Irish police found £2.3m stuffed in a wheelie-bin at the home of a suspect. The Tonbridge gang will have had to work out what to do with 800,000 pieces of paper, weighing up to 900lbs.

There are ways to launder that volume of cash, despite the ferocious laws introduced recently to track and stop terrorist money, but to do so will take expertise, time and patience. Vast amounts of patience. Far more patience than bank robbers and their accomplices usually have, however professional they are. Somebody usually cracks.

Gang members get overheated - petty arguments are magnified when nerves are twanging like guitar strings - and jealousy can also lead to betrayal. Or somebody might have a friend, lover, mother or son who wants a slice of the £2m reward offered by police. Greed can be stronger than love or blood.

Failing that, there is always a muppet: somebody on the fringes of the gang who does something really stupid. That is what appeared to have happened on Thursday when a 41-year-old woman went into a building society in Bromley, not far from the raid, and tried to open an account with £6,000 in cash - all bound up in tape marked "Tonbridge".

She was arrested. So were a 29-year-old man and a 31-year-old woman on the same day, elsewhere in south London, on suspicion of conspiracy to commit robbery. But all three were released on bail, and the 41-year-old told a reporter yesterday that she had got the money from another building society.

She may well have received it in good faith, but history says that even the most meticulous bank robbers are capable of stupidity. Mickey McAvoy, co-mastermind of the £26m gold robbery at Heathrow in 1983, moved from a council house into a mansion, bought two Rottweilers and called them Brinks and Mat. He was sentenced to 25 years in prison.

Property used to be a good way of spending stolen money, as did cars and paintings, but now estate agents are obliged to report any suspicious cash offers. They, as well as bankers, car salesmen, art and antique dealers and others, face five years in prison for allowing the proceeds of crime to be laundered. Some casinos are less scrupulous. "I am sure there are also traders in gold and jewellery who will accept cash and will not be particularly worried about where it came from," said Professor Sikka. "There is still a culture in which a lot of cash changes hands."

But £40m is a vast sum, and Britain is a difficult place in which to move large amounts of dodgy cash these days. After 9/11, the Government increased the pressure on financial institutions to report unusual transactions. The 2002 Proceeds of Crime Act means that law enforcement agents can seize cash over the value of £5,000 if the owner cannot provide evidence that it was earned legitimately. The Assets Recovery Agency has seized £100m in the past year, in a series of raids. One assistant chief constable said the new powers were "as significant a breakthrough as DNA".

So making this money clean will mean getting it abroad. The discovery of the van at Ashford suggests that the gang may have done that quickly, before the ports were notified of the raid. The registration numbers of their vehicles would have been recorded, and police have been checking the CCTV in the Channel Tunnel, but the plates are likely to have been false - as are the ID of the driver and passengers.

The cars could then be switched for others in France and left burning, like the Volvo saloon that was used to kidnap Mr Dixon and then set alight in a village near Maidstone. A red Vauxhall Vectra was also found burning close by. Mr Dixon's silver Nissan Almera had been parked at a pub. The red van with Parcelforce graphics on the side, which may have been used to abduct his wife and son, was abandoned at another village.

If the gang did cross the Channel straight away, their aim should have been to reach some place where customs inspectors could be bribed into giving a certificate to say that the origin of the money had been explained, so that banks would take it without question.

"They will look for countries in a state of flux," said John Horan, a money-laundering specialist with the accountants Harbinson Mulholland. "The Balkans would be a good place to start." The IRA planned to invest €20m of stolen money in converting a Bulgarian former Soviet military base into a luxury hotel, until a series of police raids thwarted the plan.

But what if the gang is still in the country? Once they have waited, an alternative method of changing currency would be to use "smurfs", trusted lower-level criminals. They will be used to dealing with funny money - it's not as if these gangs will never have seen it before - but will not necessarily know where the notes are coming from on this occasion. If any of them guesses, there may be trouble. Bundles will be taken to friendly bureaux de change and swapped for euros. This will take time, because the transactions will have to be under £5,000, in order to go unrecorded.

Customs officers have cut deals with bureau owners in the past, turning a blind eye to some activities as long as other suspicious transactions were reported. Nevertheless, there is a history of small London bureaux being used to launder money.

One owner known as "Sammy the Kurd" was jailed in 1999 for laundering £70m through his bureau in Notting Hill. Ussama El-Kurd was fined £1m, in addition to getting 14-year prison sentence. The money was thought to have been used by Liverpool gangs to import drugs from the Netherlands.

Once converted into euros, the money may well be taken to a country such as Liechtenstein, where strict secrecy laws protect the identities of bank account holders. Around 75,000 companies are based there - at least on paper. The government there has signed up to new regulations against tax evasion, but it remains a base for Russian and Italian mafia activity. One of the Brinks Mat robbers also opened an account there (for a company called Moyet after his favourite champagne. He spelt it wrong).

The next step will be to set up companies in Liechtenstein and in a Middle Eastern country such as Dubai, whose reputation as a new financial centre is based on not asking awkward questions.

One fake company will now buy products from the other. The products may not exist, or for authenticity's sake they may just be hugely overpriced - one American company was caught buying toilet rolls for $1,000 a time from a sister company - but the money that is transferred will be real. From Dubai, it will be moved on, through a series of similar transactions, to an offshore haven such as the Cayman Islands.

The police may be able to follow the money trail, eventually. New legislation and international agreements formed as part of the campaign against terrorists means the British government has greater access than ever to accounts held abroad. But the legal systems in the places the robbers might use are deliberately slow in providing investigators with information about the flow of cash through accounts. Each step along the way could delay the police by six months - and finally, in the Caymans, they may run into the sand.

The law there does not require companies to say who is benefiting from their activities. Professor Sikka says that they can be set up online, and need not have any physical presence on the islands. "Secrecy is guaranteed by legislation."

And how would the cash be brought back from the Caymans? Not in suitcases, by big men wearing loud shirts and smoking cigars. That's the movies.

Instead, a "stored value" account would be used. This is a bit like a prepaid phone card: it is charged up with money from elsewhere, then an anonymous debit card is issued (and sent on to a postal address), which can be used in a hole-in-the-wall machine anywhere in the world, without the identity of the user being traced.

After a long, nerve-shattering and complicated process in which the robbers might be betrayed, found out or cheated at any step, they can finally flash a little cash. The loot would be enough to buy 160 Rolls-Royce Phantom cars. If they have not already gone to prison, killed each other or been driven mad.

How to launder £50m

COUNT THE CASH. If there are any new, traceable notes, chuck them. Prepare for prison. What we are about to describe is theoretically possible, but the chances of success are extremely slim

BURY IT, put it away, stick it in an inconspicuous lock-up or security vault for as long as you can bear to do so, at least until the fuss dies down. That might take a year

HIRE YOURSELF A RELIABLE FINANCIAL ADVISER who knows both how to bend the rules and keep his mouth shut. Pay him well. (This is not a real one, by the way)

SPEND A LITTLE IF YOU MUST, but keep it to small amounts of used notes only, in places such as dodgy casinos where nobody looks too closely at the cash. Don't be flash

RECRUIT SMURFS - lower-ranking villains who are used to handling funny money - to change bundles of less than £5,000 for euros at friendly bureaux de change

COLLECT THE EUROS and take them into Europe, through the Tunnel or on the Eurostar. Stay calm and watch your mates carefully - there is a £2m reward out, after all

WALK INTO A BANK IN LIECHTENSTEIN, one of the few countries in Europe where the law protects the identity of account holders, even from police investigators

OR SMUGGLE THE NOTES OUT OF BRITAIN. If you did not do this on the same night as the robbery and before the alarm was raised, then wait as long as you can

TAKE THE MONEY TO A COUNTRY where the customs officials can be bribed to certificate your cash as clean. Start a company. Experts suggest the Balkans would be a good choice

START ANOTHER COMPANY IN DUBAI, where directors do not need to be named. Use the Balkan money to buy expensive products from it (which need not exist)

TRANSFER THE INCOME from Dubai to a tax haven such as the Cayman Islands where secrecy is assured. Set up a blind trust and nobody will know it's for you

BRING THE MONEY FROM THE CAYMANS back to the UK through more shell companies. Set up an apparently legitimate business here. And hope nobody asks why it never goes bust

OR USE THE PROCEEDS you have brought back to buy - slowly, discreetly - diamonds, property or anything else that might be an investment. Then sell up, and all your money is clean

OR CHARGE UP an anonymous 'stored value' account in the Caymans and withdraw money, untraceably, from any ATM. In your dreams. In reality, you will probably be in prison by now

THE USUAL SUSPECTS

Mister 'R'

Leader of a south London crime syndicate, one of three seen by the police as main suspects for the Tonbridge robbery. The 51-year-old R lives just 20 miles from where the raid took place. He is believed to have organised a £124m cocaine delivery into London in 1994, which was ambushed by police. Also said to have planned the Millennium Dome diamond raid of 2000, when robbers were arrested making the attempt. Connections to gangsters in Italy and Colombia. "Cross him and you wind up dead in a Kent ditch," says a London gangster.

The A Team

Six brothers are the main figures in south-east London's top crime family. Long-standing relationship with R. History of armed robbery and serious crime, although flourishing drugs trade makes them less likely to have carried out the raid. Legitimate cover includes restaurants, pubs and car dealerships. One brother is in prison for heroin importation. "They are masters of finding an 'insider' who tells them all about the routines of a security depot and who has the keys," says a former policeman. "They are said to have once dropped an informant into a vat of acid."

The Grandfather

A violent south London criminal, involved in a major armed robbery two decades ago that may have been the model for the Tonbridge heist. His syndicate weakened by recent trials, he has lately been keeping out of Britain. Noted in younger days for wearing a trilby over his balaclava during robberies. His brother-in-law specialised in infiltrating security companies to get insider information. "The Grandfather is the most frightening man I have ever met," said one former robber. "He is totally immoral and capable of any level of violence."

Paul Lashmar