The offence is alleged to have taken place near Revolution bar in Call Lane, on Sunday.
Police said the 25-year-old woman was in the area at about 4.10am.
And today, officers are appealing for witnesses to get in touch, and have issued CCTV pictures of a man they want to speak to.
He is described as black, with short hair and stubble on his face.
He is aged in his late 20s, of medium build and approximately 5ft 7ins tall, and was wearing a dark-coloured coat with fur around the hood and dark-coloured trousers.
Officers are also trying to trace the two other men pictured in the second image.
Any witnesses or anyone with information about the incident is asked to call police on 101, quoting crime reference 13170553154