LONDON — They should have listened to ‘The Guv’nor.’

The masterminds behind the biggest diamond heist in British history were only caught because greedy members of the gang refused to stick to the plan that was approved by Brian Reader, 76, the former right-hand man to one of the most notorious gangsters in London.

Scotland Yard detectives failed to trace the getaway vehicle; no fingerprints were discovered; CCTV footage was wiped; and no witnesses saw the theft of cash, gold and jewels worth $20 million.

If the gang of aging criminals had remained true to their original plan for one last job, which had been painstakingly honed over more than a year, they might still be enjoying their retirements. Instead, they can expect to spend the rest of their lives behind bars.

The conclusion of the trial of the men accused of being involved in Britain’s biggest burglary came on Thursday, when three accomplices were found guilty and one man was acquitted.

The ‘Hatton Garden Raid,’ which took place over last Easter, saw thieves steal the jewels from safe deposit boxes after boring into a vault.

Detective Constable Jamie Day, who was the first officer at the scene from Scotland Yard’s elite Flying Squad, said they had cracked the silence around the break-in by identifying a car that was never supposed to have been involved in the heist.

A white Mercedes was used briefly to observe the scene before some of the villains returned to the Hatton Garden Safety Deposit for a second night—against the wishes of Reader.

Reader, who was linked to the killing of a police officer in 1985, demanded that the gang walk away once a piece of equipment had failed on the first night of the raid. His comrades, who would later call him an “old ponce” for refusing to go back, improvised the use of the car without his approval.

The Mercedes was not registered to any of the criminals but number plate recognition cameras set up around London gave detectives a pattern to follow in the weeks after the diamond raid. “If a vehicle goes from A to B at about the same time every day, all you have to do is wait for it,” Day told The Daily Beast.

Once they were on the trail of the car, they had effectively identified the first member of the gang—John Collins. They later placed surveillance devices inside that car, and a second vehicle. They could scarcely believe their luck when the gang proceeded to brag about their record-breaking haul.

After one Friday night spent eating fish and chips and drinking in The Castle pub in Islington, Reader caught the Tube and the others got back into the cars.

“On the way home, having had a little bit more alcohol, they were a little bit more fluid with their opinions,” said Day. “That’s obviously quite useful for us as well.”

With Reader safely out of earshot, two members of the gang said they were going to confront the old gangster for declining to return to the safe even though they had not managed to steal anything on night one.

After successfully breaking into the building, climbing down a lift shaft, and drilling through the 20-inch cement wall of the safe, they fell at the final hurdle when a pump failed as they tried to punch through a wall of safety deposit boxes.

On the second night, the smaller band returned with more equipment—and left with cash, gold, and diamonds worth $20 million.

They were convinced that they had proved Reader wrong.

“You gave up being a thief 10 years ago, you cunt,” said Terry Perkins, 67, imagining the confrontation.

Daniel Jones, 60, joined in. “He was a thief 40 years ago, they never took no chances, had it all their own way. Like all them thieves then,” he said.

These sentiments would never have been shared with Reader. At least, not until they were played out loud in a South London court.

Reader may be in his mid-70s, but he is a fearsome character.

After the conclusion of the trial, it can now be reported that Reader was once a close associate of Kenneth Noye, a savage gang boss who is serving life in prison for murder.

In 1985, a police surveillance officer who was following Reader and Noye was stabbed to death. Detectives John Fordham and Neil Murphy were watching the men because of their suspected involvement in disbursing the proceeds of the legendary Brink’s Mat armed robbery in which cash and gold worth $35 million were stolen from a warehouse near Heathrow airport.

Murphy would later tell The Guardian that Reader kicked his colleague as he lay dying on the ground. Noye had stabbed the officer 11 times. Both men escaped jail when the jury accepted Noye’s plea that he had acted in self-defense—not knowing why the plain-clothed officer had snuck onto his property.

Reader was later jailed for nine years for his part in the Brink’s Mat robbery. Noye was convicted of murder for a separate fatal stabbing in 2000.

Such was Reader’s criminal expertise that the other members of the over-the-hill mob referred to him as ‘The Guv’nor’ or ‘The Master.’

The others were no criminal apprentices, however. They all had long criminal histories. Perkins, a former candy store owner, was involved in another of Britain’s most notorious robberies.

He was sentenced to 22 years imprisonment for the Security Express robbery that was planned in an old East End boozer called The Fox. The pub was owned by the infamous Knight brothers—one of whom was married to Carry On movie star Barbara Windsor.

Bank notes worth more than $8 million were stolen from the East London depot, but Perkins and two of the Knight brothers were eventually apprehended and jailed.

At a Scotland Yard briefing, Flying Squad Detective Chief Inspector Paul Johnson said it was no surprise to him and his colleagues when the white Mercedes led them to Perkins and Reader.

“You get this repeated pattern,” he said. “All they know is armed robbery and you always get a pattern when that sort of person crops up on another job... The same people; the same faces.”

Sentencing for Hatton Garden will begin on March 7. ‘The Guv’nor’ must be wishing he’d chosen a different set of faces.