The 39-year-old woman was assaulted in the alleyway between South Holme Court and Nichols Court in Thorplands early on Saturday morning.
The incident happened at around 2.50am when the woman was grabbed by a man and pulled to the floor. The victim hit the offender with her umbrella and managed to get away.
The offender is described as tall, bald, about 60 or over with a short grey beard, blue eyes and a green or brown hooded jacket.
Officers would like to speak to anyone who witnessed the incident or who may have any information about it.
Witnesses or anyone with information can contact police on 101 or Crimestoppers, anonymously, on 0800 555 111.