With the weekend in their sights, a pair of Securicor drivers hopped into a van carrying £9 million for their final rounds of the week.

They passed morning commuters as they wound their way through an industrial estate near Maidstone. But no more than 500 metres in, a blue Ford Transit screeched to a halt in front of them, while a truck blocked them in from behind.

Moments later, an articulated lorry with a spike fixed to its back crashed into the security van’s rear doors. The nine raiders attempting to break in were after the cash inside.

During the maelstrom that ensued, two police officers arrived, gunshots were fired and the masked robbers fled, thwarted. They were driven to a speedboat less than a mile away, which carted them along the River Medway, away from the glare of the law.

Dozens of officers joined their colleagues at the scene; among them was Detective Sergeant Ian Dampier.

He watched as members of the bomb squad tentatively approached suspected homemade mines that had been attached to the security van in a bid to frighten the drivers.

Rather than being explosives, the bemused officers realised they were Fray Bentos Steak and Kidney Pie containers obscured by a coating of paint.

“They were covered with flashing LED lights, so they looked like explosive devices. The tins caused problems because we had the whole area cordoned off until the explosives team came in and had a look,” the 61-year-old, who has since retired from the police, remembers.

“The Securicor vehicle had nowhere to go. Once it stopped, the robbers drove over in a lorry that had a huge metal spike concreted into it. The idea was you’d stick it through the rear doors, then put a rod through and pull them off.

“As the robbery progressed, police got called by witnesses. There were two shots fired and they made good their escape.

“I hadn’t really seen an operation as coordinated as that. It was very old-fashioned, really. We don’t get many with that level of planning; they’re generally much less sophisticated.”

The events would have fitted seamlessly into a Michael Mann film – yet they took place in Aylesford on July 7, 2000.

Forensics teams managed to lift saliva from gloves in the getaway van. Bullets were salvaged from the scene and the vehicles used were found to be stolen, with counterfeit tax discs – all of which were produced in a similar way – stuck to their windscreens.

All but two of the criminals’ motors had petrol bombs taped up inside; they intended on torching them to destroy any forensic evidence.

But it was the words daubed on the girders of the makeshift battering ram that attracted the attention of the Metropolitan Police’s Flying Squad, previously nicknamed The Sweeney.

“There was a bit of a clue painted on there. There were the words ‘GERTI Mk II persistent, aren’t we?’ It was a bit of a calling card,” Ian says.

“That upped the ante for us because it’s not every day that you work with the Flying Squad. I only did that twice in my 33 years as an officer.”

Gerti – the meaning of which remains a mystery – had been discovered at another Securicor raid in Nine Elms, London, five months before. As in Aylesford, the raiders used a metal spike in an attempt to tear through a security van’s doors, behind which lay £10 million. But, again, they were forced to abort the operation and escaped empty-handed.

Despite making one arrest in connection with the summer raid, detectives made slow progress with their investigation. The only lead they had was provided by a source who told them a person called Terry was involved in the heist.

That was until a man drunkenly lost control of his car and drove into a pub in Horsmonden, near Tunbridge Wells, on August 17. Its driver was known villain Terry Millman.

“He was an armed robber,” Ian adds. “The vehicle he was in was stolen and had the same forged tax discs as the ones used in the Aylesford robbery. That was a crucial moment. There was a lot of activity after that because we’d found a main player in this gang.

“He was a character. He was an old-style criminal, incredibly loyal and probably a bit of a liability.”

Millman's fingerprints matched three found during the Aylesford raid. Detectives also discovered that he had regularly travelled to Kent in order to visit the Wenhams, a family of horse traders, at their farms in Horsmonden and Brenchley. “They mixed with some quite highfalutin people, and they would have seemed like lovable rogues," Ian says.

Identifying Millman as a potential link between the two robberies, the officers, in Ian’s words, “let him run and watched where he went”.

Covert cameras had months before been placed in discreet locations surrounding the farm after police started to suspect Lee Wenham of criminality. While reviewing the tapes, officers realised they had captured footage showing the lorry with the concrete spike leaving the Brenchley plot on the day of the Aylesford robbery.

“The cameras saw lots,” Ian chuckles. “We’d been watching him for months. We had people stick a tape in the camera, come back to change it and then review it – so we didn’t know about all of the activity on July 7 until we reviewed the video.”

Police believed those involved in the two failed plots – which were both costly and time-consuming to organise – would be readying themselves for another job. And the following month, Wenham was observed visiting the Millennium Dome. Having travelled from Kent to the Greenwich venue, he spent just six minutes inside.

“He was literally there for about six minutes and he went to the MoneyZone. There was an awful lot of conjecture amongst us about why on earth he’d do that, and then some genius said there was a diamond in there. That was the first inkling we had that they were wanting to steal it.”

A De Beers diamond exhibition was on display inside the south east London site. The collection was worth about £350 million and included the priceless Millennium Star. If the robbers were successful, they would have undertaken one of the most audacious heists of all time.

A detective based at the Dome also spotted by chance two known criminals - Raymond Betson, who owned a luxury home in Chatham, and Catford’s William Cockram - filming the entrance to the MoneyZone and the jewels on display. They later met a third man there, Aldo Ciarocchi.

Betson, now in his late 50s, notched up his first conviction at the age of 14 for burglary. He reportedly made £15 million from thieving and drug deals.

Suspecting a raid was in the offing, police wanted to catch the gang red-handed. The Millennium Star was moved to De Beers' London headquarters and replaced with a worthless fake. By this point, surveillance officers at Tong Farm in Brenchley had also noticed the arrival of a speedboat that had been bought by Millman, who used the alias Mr T Diamond, in Whitstable.

“We thought it was going to happen on a number of occasions,” Ian continues.

"I think on one day the JCB actually got involved in an accident on the way to the Dome.

"They also got the tides wrong once. It wasn’t a seamless job by any means.”

After 8am on November 7 - 20 years ago today - the gang's speedboat was spotted in the Thames. A JCB left a coalyard near the Dome and trundled towards the venue, its digger raised. With four members of the gang on board, it flattened the perimeter fencing and ploughed through the wall of the Dome.

The raiders - who were wearing gas masks - then used a nail gun and sledgehammer to smash the glass display case before releasing ammonia and several smoke grenades into the air.

But the robbers were overpowered by dozens of armed officers within inches of the jewels. In all, more than 200 Metropolitan Police detectives, many of whom were disguised as bystanders and cleaners, were involved in the operation, codenamed Magician, which will be the subject of a Ross Kemp documentary on ITV on Wednesday.

More London officers stationed outside the venue arrested two other suspects – one waiting in a high-powered boat on the Thames and another thought to be tracking radio frequencies. No shots were fired.

Ian was in Kent when he learned of the bust. “There was a certain amount of excitement because we’d been waiting for this for months.

“There was chaos after that, though, as there were all the arrests that then took place, searches and getting the evidence together. It was a joint-operation.

“It’s always been seen as the Flying Squad’s job, but we did a massive amount of groundwork. There’ve been numerous books written about the Dome and I haven’t seen us [Kent Police] in any of them; we’d almost been forgotten about.

“One of my colleagues describes this as being a 400-metre relay; we did the first three legs, then the Met ran the last and ran off with all the medals.”

Six more men - including Wenham - were arrested the following morning in Horsmonden and the village of Collier Street, near Maidstone. During raids of the Wenhams’ farms, Ian says it became apparent that the sites allowed “for all of the ancillary stuff to take place”. They found laundry bags, which were believed to be intended to get the money away from Aylesford, reflective coats, a newspaper dated August 6 bearing Millman’s fingerprints, shotgun cartridges, tabards, and a Smith and Wesson revolver.

Robert Adams, of no fixed address, and Ciarocchi, from Bermondsey, were sentenced to 15 years each following a trial at the Old Bailey.

Millman, 57, died of cancer in November 2001, before the trial took place.

Wenham was ordered to serve nine years in prison for conspiring to steal the gems and his part in the Aylesford plot. All charges against his father, James Wenham, were dropped.

Kevin Meredith, the speedboat driver, was jailed for five years after being found guilty of conspiracy of steal.

And Betson and Cockram, who have long-been considered the masterminds of the raid, had their 18-year sentences later reduced to 15. However, Betson was jailed again in 2014 for 13 years following a bungled security depot ram raid in Swanley.

“I think money drove him. They’re big rewards, aren’t they?” Ian explains. “You do get career criminals and for some it’s a way of life, while others stop.

“They got caught, never really whinged about it and those who got convicted did their time. There wasn’t a love-hate relationship between us and them; it was all in fairly good spirits generally. They were old-style.

“Imagine the planning that goes into this; if it all goes wrong you get nothing and you get 17 years. There’s got to be easier ways of doing that with less risk, like cybercrime, and I’m sure that’s why we don’t get as many as we used to.”

Read more: All the latest news from Kent