Robert Powell, 50, was shot dead after the celebration in Roydon in the early hours of June 13, 2020, following a plot police described as rare in its brutality and organisation.
He was shot eight times. Two women were also injured.
Despite the refusal of anyone present to talk to police, two people have since been convicted of murder.
Temitope Adeyinka, 41, was found guilty of manslaughter on Friday, November 7 following a trial at Woolwich Crown Court.
He was cleared of murder and possession of a firearm.
Adeyinka, who went by the street name ‘Limo’, was a long-term associate of Nana Oppong, 45, who was named as the organiser of the attack.
Oppong and another associate, Israar Shah, 41, were convicted of murder last year and sentenced to life in prison.
Oppong, a high-level criminal with a history of violence, use of firearms and drug dealing, had long been set on killing Mr Powell over an unknown grievance.
Learning that Mr Powell would be attending the party, he formed a plan with a team of co-conspirators.
Nana Oppong led the plan in a Ford Kuga – unregistered and used solely for the murder plot – driving to the scene with the support of two other vehicles, Shah’s Toyota Prius and a Vauxhall Zafira.
Adeyinka, alongside Shah, was in the Prius.
This car was seen prowling up and down the road outside the party venue, the pair keeping watch for Mr Powell.
When Mr Powell emerged, the Kuga carrying Oppong pulled up near him, a gunman briefly stepped out and the murder took place.
CCTV, telephone and vehicle tracking evidence proved Adeyinka met up with Shah and Oppong, travelled to the party and was in contact with Oppong in the hours before and following the murder.
Enquiries continue to identify others involved.
During their trial in May last year – and following an intensive four-year manhunt carried out in partnership with law enforcement abroad and the National Crime Agency – Adeyinka was tracked down in Spain and arrested.
Detective Superintendent Stephen Jennings said: “This has been a complex, difficult and lengthy investigation.
“It became clear as our enquiries began, in the aftermath of the party in Roydon, that not one person in attendance was willing to give a statement to us.
“Undeterred, we grew our investigation from a single piece of intelligence, piecing together the wider conspiracy to murder Robert Powell and identifying three key suspects.
“The murder of Robert Powell was rare in its brutality and level of organisation.
“We were committed – and remain committed – to bringing all of those involved in this shooting to justice.”
Samantha Woolley, a specialist prosecutor for the Crown Prosecution Service, said: “This has been one of the most challenging and complex cases of my legal career and involved extensive liaison with the CPS Extradition Unit to bring Temitope Adeyinka rightfully back to the UK to face justice.
“Each and every one of us involved were determined that those responsible for the murder of Robert Powell were brought to justice and we worked tirelessly to achieve this.”
Gill Duggan, head of Europe at the NCA’s international command, said: “Adeyinka’s conviction is another superb result and shows once again that UK law enforcement does not give up on finding those who have fled justice in the UK.”
