They say an attack took place outside Gaumont Place at 5.30 on Tuesday morning.
While it was not reported at the time, it was later seen on CCTV footage and police were alerted the following day.
Detectives are looking for a black man, of medium build and in his early 30s in connection with the incident.
Specialist officers are also appealing for the victim to contact police.
Detective Constable Charlie Codrington, who is leading the investigation, said: “We know the woman in this footage will be scared, but we urge her to contact the police. We are concerned for her wellbeing and officers are here to provide her with welfare, medical and trained support.
“This was a horrifying incident which took place in broad daylight, close to a busy high street. We have released images of a man we need to trace and would urge anyone who recognises him to contact us.
“If you have any further information about what happened, please come forward.”
Anyone with information that could help police should call 101 quoting 01/7556597/26, or Crimestoppers, anonymously, on 0800 555 111.
