Four government spies were quartered near the residence of Karl Marx. One was his next door neighbor, one a shopkeeper, and two lived across the street, whose presence is entirely unknown to the neighborhood. They reside in an attic with two windows that provide an excellent vantage point to view Marx's residence.
The four men were all government trained special agents, capable of executing any target quickly, silently, and obviously (when required). They had been surveying Marx since he arrived in London, with the fears of the Prime Minister being he would try and lead a communist movement in London, as he had in other capitals around Europe.
The men, however, did not only follow the orders of the Prime Minister. After all, they were Her Majesty's Own Secret Service. Since they had been stationed there, they were given specific orders by Victoria: should a hooded man ever approach the residence, kill him on sight.