The man, who has not yet been identified, was last seen alive at 20:45 BST on Tuesday 9 June in St Stephen's subway.
Two people saw the man, 29, in the early hours of Wednesday before he was found dead at 07:00 BST, police said.
A post-mortem examination revealed he died from injuries to the spleen and the death was declared "suspicious".
Det Insp Marie James said: "We are still trying to create a timeline of the man's final movements and the circumstances which led to his death.
"We would particularly like to hear from two people who saw the man in the early hours of Wednesday morning.
"Other witnesses have suggested a male teenager, possibly a college student, passed him in the subway at around 05:00 BST and later returned to buy flowers from the flower stall by the subway to leave inside.
"The second person we wish to trace is a man who gave assistance at the scene when his body was discovered.
"This man is described as being smartly dressed, wearing a jacket and glasses.
"I would also appeal again for people to contact me if they know details about any of his friends or family as we have still been unable to trace next of kin despite numerous inquiries."

Manchester United trainee Rothwell, 21, has penned a two-year deal.
The attacking midfielder made seven appearances in all competitions during a loan spell with Barnsley last season.
Maguire, 27, has signed a one-year deal after playing 25 games last season during a loan spell and subsequent short-term move from Rotherham.
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The 27-year-old will not need surgery after breaking his leg in an innocuous clash with Reds winger Raheem Sterling.
Latics boss Roberto Martinez said: "It is not as bad as it could have been."
Watson, who spent two nights at the Royal Liverpool Hospital, has now returned home to begin his recovery and will wear a cast for a few weeks.
The club are optimistic he will be able to resume training in the new year.
Martinez added: "It's still a very serious injury and we need to ensure that Ben is given all the time that he needs to make a full recovery.
"The good news is that he won't be out for as long as we initially thought and, with the character that Ben has, he will be able to deal with such an injury."
Former England Under-21 international Watson collapsed to the Anfield turf clutching his right leg shortly after the half-hour mark of Saturday's Premier League match.
But he waved away the help of a stretcher and limped off with the support of Wigan medical staff.
Watson, who joined from Crystal Palace in a £2m deal almost four years ago, had been in and out of the side this season but had started the last three Premier League games.
Martinez said: "Ben has just started to put his stamp on the team and he will be missed in the group because of the type of lad he is."
Police investigating the death of a homeless man in a Norwich subway want to speak to two potential witnesses.
[NEXT_CONCEPT]
Oxford United have signed midfielder Joe Rothwell on a free transfer and agreed a new contract with striker Chris Maguire.
[NEXT_CONCEPT]
Wigan midfielder Ben Watson could be back in training within six weeks despite suffering a fractured shin in the 3-0 defeat at Liverpool.