A 52-year-old man was walking along Arundel Gate around 2pm on June 10.
He was approached by a man claiming to be a police officer, who demanded £65.
When the victim refused, he threatened him with a knife and stole his bag of groceries.
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Police would like to speak to this man.
Officers are keen to identify the man in the image.
Contact 101 with any information, quoting crime reference number 14/73538/20.
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