Staff were locking up the store in Dunkirk Lane when three men ran towards them and forced them back inside at around 10.10pm on Saturday (November 13).
Armed with weapons, including a hammer and a kitchen knife, the men ordered staff to put a "significant amount of cash and cigarettes" into a duvet cover.
The robbers then forced the staff to walk with them across a nearby park to ensure they did not call the police before they could escape.
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Although none of the staff were injured during the robbery, detectives said they were subjected to "continued threats of violence".
DC Laura Lawson, of South CID, said: "This was a shocking incident committed against staff who were preparing to go home after a hard day's work.
"Instead of being allowed to do that they were instead subjected to a terrifying robbery involving weapons and threats of violence.
"Fortunately, none of them were injured but they were understandably left extremely shaken."
The first man is described as white, around 6ft tall and of a stocky build.
He was wearing black clothing with a black ski mask as well as red gardening style gloves and a brightly-coloured drawstring JD bag.
The second suspect is described as a white male, of skinny build, with blonde hair and distinctive front teeth which are bucked to the side.
He was wearing a two-tone grey and black Under Armour top, grey tracksuit bottoms with white markings, green trainers and a black balaclava.
The third man is described as white and of average build and height.
He was wearing all black clothing including a black Adidas sweatshirt, black gloves and a black balaclava.
No arrests have been made at this stage and enquiries are ongoing.
"We are now asking anybody with information or anyone who recognises the suspects in this case to contact police as soon as possible," DC Laura Lawson added.
"If you have any concerns or you have any information which could assist our enquiries then please stop an officer and say hello.”
Anyone with information can contact police by calling 101 or emailing [email protected], quoting log number 1743 of November 13.
Alternatively, independent charity Crimestoppers can be contacted anonymously on 0800 555 111 or online at crimestoppers-uk.org.
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