The man is described as around 6ft tall, broad, wearing a dark top with the hood up, a dark mask, dark trousers and light-coloured trainers.
Officers investigating the incident are now asking for a significant witness, who may have information about an incident, to come forward.
The victim walked past a dog-walker – described as a woman wearing a bobble hat, possibly blue in colour, walking a black and white dog. Police want to identify and speak to this dog-walker to see if she saw anything, or saw a man matching the description provided running down Landguard Road.
Inspector Rob Abel said:
“Officers will be carrying out enquiries in the area, but we will also have officers conducting patrols over the next few days. If you see us and have any concerns, or information, do not hesitate to approach us.
“We understand there were vehicles passing through the area at this time. We would like to hear from anyone who drove through this area, including the lower part of Regent Street approaching the train station, between 4.45pm and 5.30pm and perhaps saw the man involved in this incident or captured anything on Dash Cam.”
Anyone with information is asked to contact police on 101, quoting 44210014346. Alternatively, you can contact the independent charity Crimestoppers 100% anonymously on 0800 555 111, or via their anonymous online form at crimestoppers-uk.org