Between 7.30pm and 7.50pm on Saturday, February 25, a man carrying a knife entered the Tesco petrol station on Sizer Way and demanded cash from the till operator.
The cashier opened the till and gave the man some money. Around £400-500 was taken.
The man then left the shop while a second man held the door open for him. They both fled on foot in the direction of the roundabout at the entrance of Tesco.
The first man has been described as aged approximately 20 to 30 years, slim, around 5ft 11ins tall, and spoke with a local accent. He has been described as wearing a black jacket with a dark grey hooded top underneath with the hood up, a light coloured face covering which went to the top of his nose, and tight white gloves.
The man who held the door open was described as wearing dark clothing.
Officers would like to hear from anyone who saw two men matching this description either entering or exiting the store, or acting suspiciously in the surrounding areas near to this time.
Anyone with information is asked to contact CID West on 101, quoting 44170073345, or call Crimestoppers anonymously on 0800 555111.