Why was a spy sent to St. Antoine? Why did he visit Defarge wine-shop?

British spies, John Barsad ( Solomon Pross) and Roger Cly, are trying to frame French émigré Charles Darnay for their own profit. John Barsad visits the wine-shop to question the Defarges about the discontent in Saint Antoine because of Gaspard's execution. 

the spy was John Barsad he visited the wine shop to take more information about the revolution and he told Mr Defarge that charles Darny that lucie is going to marry is the son of evermonde.