Kyrone Moodie was found injured in New Park Road around 9pm on September 8.
The 27-year-old was taken to hospital for treatment where he later died.
Officers have already charged three men with murder and they remain in custody.
However, police believe several witnesses have yet to come forward, and there may be additional people of interest.
Detective Chief Inspector Sarah Lee, who is leading the investigation, said: “While my team has made significant progress, we are now appealing to the public to help ensure that everyone responsible for this tragedy is held accountable.
“In particular, the residents of New Park Road may hold vital information that could help us identity more people involved.
“Were you in the area on Monday, 8 September? Did you see or hear anything suspicious?
“A reward of £20,000 is now being offered to encourage the public to speak up and help us.”
To provide information to police directly, call 101, giving the reference CAD 7805/8SEP2 or Operation Concito.
To provide information anonymously, call Crimestoppers on 0800 555 111.
Kyrone’s family continue to receive support and updates from specialist officers.
