An appeal for information about the murder of James Attfield will feature on BBC's Crimewatch tonight.
Detectives are still trying to establish all of Mr Attfield's movements between leaving the River Lodge pub shortly after 10pm on Friday March 28 and when he was found dying from stab wounds in Lower Castle Park shortly before 6am on Saturday March 29.
Police want to trace a woman who is seen in CCTV walking through the park (image 7, previously released) at 12.36am on Saturday March 29 and then again with a man at 2.19am (image 14, previously released).
Officers are also still trying to trace a couple who were seen sitting on a bench in Lower Castle Park close to the lake at 11.30pm on Friday, March 28.
The man is described as white, of stocky build and appears tall with long legs. The only description of the woman is that she is white. They were sitting at opposite ends of the bench and it is not certain if they know each other, although they may have been a couple who had had a row earlier in the evening and were sitting apart not talking to each other.
Approximately 10 metres away from where this couple were sat, there was also a man sat on the grass who was white, aged in his 30s and was wearing a dark jacket and possibly jeans. This may, or may not, have been James Attfield.
A total of six people caught on CCTV walking through Lower Castle Park late on Friday March 28 and into the early hours of Saturday March 29 have now been identified and eliminated from enquiries.
A further 17 people have yet to come forward.
On Wednesday, April 23, a 27-year-old man from Colchester was arrested in connection with the murder.
He and a 27-year-old woman, also from the town, are now on police bail until Friday, May 9 pending further enquiries A reward of up to £5,000 has been offered by Crimestoppers for information which leads to the arrest and conviction of the person, or persons, responsible.
The show will be on BBC One at 9 oclock tonight.
Anyone with information is asked to call Essex Police on 101 or Crimestoppers anonymously on 0800 555111.