The incident happened in Norman Road between 10am and noon last Wednesday (December 8) between two men.
Another member of the public in a high visibility jacket is also believed to have witnessed the robbery and officers are asking that person to come forward.
A 37-year-old man was arrested on suspicion of assault and theft and released on bail.
Anyone with information is asked to report online or ring 101 quoting serial 731 of 9/12.