Underworld accountant and diamond merchant Saul Nahome was shot dead outside his home in Finchley, north London, on 27 November 1998.
Known as Solly, 48 year-old Nahome specialised in money laundering and was known as a 'financial adviser' to the feared Adams Family.
His murder came eight months after the disappearance of another Adams associate, Gilbert Wynter, 37. Legend has it that Wynter is buried in concrete under the Millennium Dome.
At the time the leader of the Adams Family, Terry Adams, was under police surveillance.
Detectives from the Met Police's murder squad have officially linked the two cases and in May 2011 launched a new 'last ditch' effort to solve them.
Detective Chief Inspector David Manning from the Homicide and Serious Crime Command said: "Over time, allegiances change.
"I would appeal to anyone with information, despite the time lapse and however insignificant they think it might be, to contact us.
"We are now convinced that the two cases are linked. Mr Nahome and Mr Wynter both associated with the same people and they were also well known to each other having been business associates at various times."
Mr Nahome, a Hatton Garden jeweller said to be involved in laundering the proceeds of the 1983 Brinks-Mat robbery, was assassinated as he arrived at his home in Arden Road, Finchley, at around 4.40pm.
As he got out of his car he was approached by a male wearing a balaclava and was shot in the back several times as he tried to run away.
The gunman then fired four times into his head before walking to the top of Arden Road and riding off on J-reg 125cc black motorcycle with a yellow or orange stripe down the side. The bike has never been recovered.
It is thought the suspect, a black or dark-skinned Mediterranean male, was seen an hour earlier standing by a pillar box in Arden Road.
At the time detectives described it as an "execution-style killing" because the gunman appeared to show no signs of stress or emotion as he calmly walked away from the scene.
Gilbert Wynter, a known enforcer for the 'A-Team', had in 1994 been acquitted of the murder of Claude Moseley, a former British high jump champion.
He was last seen when he left his family home off St Anne's Road, Tottenham, at 8am on 1 March 1998.
Mr Wynter spoke to his girlfriend by telephone later that day but did not use his mobile phone again or access his bank account again. He was reported missing eight days later.
The car he left in, a white Nissan Micra registration number L753 XJU, was found in Spring Gardens, Woodford, on 2 June 1998.
Mr Wynter is described as black, 5ft 11ins to 6ft tall, of muscular build with short cropped hair. When he left his family's home, he was wearing a 'Fruit of the Loom' grey tracksuit top and bottoms, a black puffa quilted jacket with fluorescent stripes and grey Reebok trainers.
DCI Manning said: "The cases have never been closed despite the lack of new information. Having reviewed both investigations and with advances in DNA techniques, we decided to launch a last-ditch effort to bring them to a successful conclusion."
Anyone with information should contact the incident room at Patrick Dunne House in Sutton on 020 8721 4054 or Crimestoppers on 0800 555 111.
'A quiet suburban street, a gangland hit and the death of Solly Who?' - Independent, 5 December 1998.
'Terry Adams: The Last British Gangster and the 'Goodfellas Tapes' - CourtNewsUK Crime Vault.
'Vanity cost gangland enforcer his life' (Gilbert Wynter case) - Guardian, April 19, 1999.
'Reign of Fear is Over' - Mail Online, 6 February 2007.