A reconstruction of the Jim Attfield murder was shown on BBC's Crimewatch.
The show traced Jim's last movements on the day before his death using CCTV caputured in Colchester town centre.
He was shown walking through town, taking £350 cash out at North Hill Post Office.
The reconstruction showed Jim visiting a local newsagent, at 10 pin bowling alley and The River Lodge Pub in Middleborough.
It then showed Jim sitting on a bench in Castle Park and said what happened next is unknown.
The reconstruction showed Jim attacked from behind and threatened with a knife.
The film showed Jim was still alive when found by a cyclist in front of the lake in Lower Castle Park at 5.45am on March 29 and paramedics battling to save him for 45 minutes.
A number of CCTV were also shown from Castle Park featuring a number of people detectives are keen to eliminate from their enquiries.
Jim's mum Julie Finch made an emotive appeal for information and said she could not understand how someone could do this to her son.
Parts of a video which Jim made for charity Headway was also shown.