Officers from the Metropolitan Police were called at 10.30pm on January 13 with reports of an incident at the store in Brentwood Road.
It was confirmed by police that members of staff had been subject to the assault by a group of youths.
According to a spokesperson for the London Ambulance Service, the workers declined treatment at the scene.
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Police are now investigating the incident.
Anyone with information that could assist police should call 101, quoting CAD reference number 7205/13JAN25.
Alternatively, those wishing to remain anonymous should contact Crimestoppers on 0800 555 111.