Just as importantly, the criminals themselves had begun to realise that conducting armed bank robberies - or "going over the pavement", to use the underworld's parlance - was far less lucrative and more dangerous than the drugs trade. As one Flying Squad detective put it: "What's the point of going over the pavement with a shotgun to rob a couple of hundred thousand pounds when you can make 10 times that from a consignment of drugs? You'll get anything up to 20 years for armed robbery - life, if someone gets killed - whereas the average drug conviction might get you six or seven."

The problem for the traditional, white, London gangsters is that the drugs business is truly multicultural. Those playing this game have to operate in a climate dominated by the Russian mafia, Jamaican Yardies, Islamic terrorists and Colombian smugglers. After a few years of trying to deal with such hardcore brutes, armed robbery might seem a more appealing prospect, even if it is less instantly rewarding. It is perhaps telling that this week's raid was perpetrated in Tonbridge in Kent, a neighbourhood to which many of the old white south London criminal families emigrated.

The robbery in Kent looks like an inside job: carrying it out would require either brilliant surveillance or the help of an employee. Yet it may be harder to dispose of the money than to steal it. Divided into £50 notes, a sum of £40 million would weigh half a ton.

When robbers carried out a raid on the Northern Bank in Belfast in December 2004, they found much of their haul rendered useless after the bank introduced a new-style note.

Speaking at a press conference before the arrests, Mr Leppard, who is leading a team of 100 investigators, confirmed that police were looking at the possibility that the gang had fled across the Channel. Officers have already seized security camera footage and computer numberplate records of vehicles going through the Channel Tunnel and on the Dover ferries after the raid early on Wednesday morning.
Mr Leppard also confirmed that detectives were looking into the possibility that the gang had inside information for an operation that had been “executed with military precision”.