The man burst in brandishing a handgun and demanded cash from two terrified women.
He then ran off up Forest Street with a quantity of stolen cash.
Police say the man was last spotted just after 3pm entering the Methodist Church grounds - but despite an extensive search by police dog-handlers he escaped.
The man, who wore a black scarf over his nose and mouth, is described as white, of slim build, aged between 25 and 30 years, about 5ft 10ins tall.
He was wearing a black baseball cap with a white motif on the front, a blue casual jacket, which zipped up at the front and had a hood with a white motif on the right arm, and blue jeans.
He was also wearing white coloured gloves, and dark sunglasses and was carrying a white carrier bag.
DI Chris Howarth, who is leading the investigation said there had been a number of similar robberies but they were neither linking them or ruling out any connection at this stage.
He said: "Anyone that is willing to brandish a handgun has to be considered dangerous. It was a traumatic experience for the staff." Anyone with information should ring 48000.
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