The robbery took place on Wednesday (January 25) between 7.35pm and 7.43pm, and police are appealing for information from the public to help find who is responsible.
In total the man took around £300, before leaving the store and turning left onto Bedford Road.
The suspect was described as a white male, 5’ 4”, wearing a balaclava and a dark-coloured hooded top with matching bottoms.
PC Pawan Nahar, investigating, said: “This was a particularly frightening incident for both the staff and the members of the public who were in the shop at the time.
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“We are committed to tackling this kind of unacceptable behaviour and would ask anyone who saw a man in the Bedford Road area at the time, or who knows who is responsible, to come forward.”
Anyone with any information is asked to contact PC Nahar via 101, or to call Crimestoppers anonymously on 0800 555 111.