From the Wikipedia page 
St. Martin's Le Grand is a street and former liberty in the City of London between Newgate Street and Cheapside to the south and Aldersgate Street to the north. To the east of the road once stood the collegiate church and monastic precinct of St. Martin's, of ancient origin. The precinct was within the City, but was not subject to its jurisdiction, constituting a liberty with the privilege of sanctuary.
According to a somewhat dubious tradition the church dated back to the 7th or 8th century and was founded by King Wihtred of Kent. It was, more certainly, rebuilt or founded about 1056 by two brothers called Ingelric and Girard, in the reign of Edward the Confessor. This foundation was confirmed by a charter of William the Conqueror, dating to 1068. The church was responsible for the sounding of the curfew bell in the evenings, which announced the closing of the City's gates. It was dissolved by Henry VIII and demolished in 1548. However, it retained certain rights of sanctuary until 1697 and, as such, was a notorious haven for malefactors. One of those who sought sanctuary here was Miles Forrest, one of the reputed murderers of the Princes in the Tower.
The General Post Office was established on the site of the monastic precinct in 1829 with its new headquarters. From here mail coaches departed for destinations all over the country. Coaches bound for the north went up St. Martin's Le Grand through Aldersgate - the first stage of the Great North Road (now the A1 road) to York and Edinburgh.
The nearest London Underground station is St Paul's Station, at the southern end of the one-way street.
St. Martin's Le Grand formed part of the marathon course of the 2012 Olympic and Paralympic Games.