The gang which planned the Dome robbery already had a buyer for the £200 million diamonds.

Police believe they were probably stealing on an order from the Russian Mafia, who in turn were acting for a wealthy collector, possibly an Arab.

Detective Chief Superintendent Jon Shatford, who helped foil the raid, said: "You just have to have someone who wants to own the Millennium diamonds, they could pay anything."

The robbery might have succeeded but for painstaking and old-fashioned detective work.

They arrested the gang red-handed after they crashed through the Dome on a JCB earthmover and smashed through the armoured glass protecting the Millennium Star and its rare sister diamonds.

One robber, Robert Adams, commented ruefully: "I was 12 inches from pay day. It would have been a blinding Christmas. I cannot believe how easily the glass went. I only hit it twice. Then that ****ing mob came in and jumped on us."

The "mob" was the Flying Squad, backed by armed colleagues. It was the squad's biggest operation and succeeded in thwarting the audacious raid.

The operation was launched after a chance sighting of the gang's ringleaders - known criminals - at the Dome two months before. Officers immediately instituted round-the-clock surveillance.

The Dome was among a number of high-security sites near rivers which the Flying Squad was covertly watching after an equally daring robbery bid earlier in the year.

That raid's target was a security van carrying £10 million near Battersea Dogs Home in Nine Elms, south London.

It was bungled by the robbers, who just managed to escape by crossing the River Thames on a speedboat.

The Flying Squad decided to put some of Britain's most notorious criminals under surveillance.

Mr Shatford said: "We knew they would strike again."

Because of the audacious escape across the water, detectives also drew up a list of possible robbery targets sited near rivers.

It included the Dome, although no one seriously believed then that anyone would attempt a raid on such an apparently secure site.

But on September 1, 2000, the Millennium diamonds were due to be moved from the Dome for a short period. Police thought if any robbery attempt was to be made, it would be while they were out of their vault and in transit.

They monitored the Dome and its precincts intensely that day. Nothing happened.

Instead Raymond Betson and his old friend William Cockram were spotted arriving at the attraction.

They had been of interest to the Flying Squad since 1996 and were strongly suspected of taking part in other major robberies.

Betson was also on the list drawn up after the Nine Elms robbery bid.

Mr Shatford said: "We knew these people were up for the business. This was the early stage of a plot. We started extensive police deployment on the Dome knowing now it would be a target.

"We never knew how the robbery would take place and catered for a number of options."

Police disguised themselves as tourists, secretly videoing visitors as they moved round the exhibition.

Mr Shatford said: "We knew the gang had to be arrested. We had 100 officers at a time catering for every eventuality, knowing at some stage the robbers would end up at the vault."

Betson and Cockram were put under surveillance.

Mr Shatford said: "It was the wettest summer ever that year. We had people living up trees and in flooded ditches. Equipment was damaged. We had significant difficulties to overcome."

On November 6, 90 armed officers were moved into and around the Dome.

Mr Shatford said: "I knew it was going to happen because I stood up and started to pace. We were in a high state of alert."

He was both right and wrong. The raid was to have taken place that day but the speedboat skipper who was to have helped them escape across the Thames said the tide was not right. It was postponed for 24 hours.

Mr Shatford said: "We wondered if we had been rumbled."

Nevertheless he kept up the full deployment of officers.

The next day, around 9.30am, they saw Betson driving a JCB towards the Dome. On board, hidden under covers, were Cockram, Adams and Aldo Ciarrocchi.

All wore body armour and had masks.

Mr Shatford said: "We had earlier discounted a JCB just charging in as it would be too dangerous."

But the raiders didn't care.

Mr Shatford said: "When I saw the JCB I was horrified. I was satisfied we had mitigated the risks and were strategically placed."

A party of Dorset schoolchildren were immediately evacuated from the building and other tourists were thinned out.

Mr Shatford said: "We could not disturb the environment too much, as the gang might suspect and go away. The public had to be around.

"The JCB came into sight. A Transit which had been following drove away and was not seen again."

Suggestions it contained a police agent who infiltrated the gang have been firmly denied by police.

An additional headache also came with a minor road accident on the road the JCB was travelling on. An undercover car was sent to whisk off those involved, rather than risk the appearance of a marked police car on the scene.

The JCB crashed through the gates, straight into the Dome and sped right up to the diamond vault.

Mr Shatford: "They got out looking like terrorists, wearing gas masks and their body armour. I expected them to be very heavily armed.

"They were carrying smoke grenades, ammonia and stink bombs to make people believe they were chemical weapons.

"Ciarrocchi threw the smoke flares while Cockram went into the vault with a nailgun, firing it into the three-quarter-inch glass. Adams followed with sledge-hammers.

"The order was given to move in. It was all over."