Officers said the assault occurred outside Richer Sounds in Church Street at around 2.30am on August 22.
No further details were released by the police.
Today (September 22), detectives released CCTV images of a man and woman they would like to speak to in connection with the incident.
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A spokesman for Lancashire Police said: "We believe these people may have information which will assist our enquiries.
"If you know who they are please email [email protected]"
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