The lost church of St Leonard Foster Lane originally belonged to the College of St Martin le Grand. The Dean and canons of St Martins used to worship at the altar of St Leonard inside that collegiate church but then built the separate small church of St Leonard in their courtyard in 13th century. It is shown on the copperplate map of 1555. We know that a new window was installed in the chancel in 1533, and that the church of St Leonard’s was repaired in 1631. John Stow in his survey of London had this to say : “On the west side of Foster lane is the small parish church of St. Leonard’s, for them of St. Martin’s le Grand. A number of tenements being lately built in place of the great collegiate church of St. Martin, that parish is mightily increased. In this church remain these monuments. First, without the church is graven in stone on the east end, John Brokeitwell, an especial re-edifier, or new builder thereof. In the choir, graven in brass, Robert Purfet, grocer, 1507; Robert Trapis, goldsmith, 1526
St Leonard Foster Lane was destroyed in the Great Fire of 1666 and not rebuilt. The parish was combined with Christ Church Newgate Street. The site of the church still had some remaining ruins which were not cleared away until the General Post Office was built on that site in the early 19th Century. The graveyard of St Leonard’s was one of the three that was used to create Postman’s Park which commemorates heroic self sacrifice.
You can find a blue plaque marking the site of St Leonard’s halfway up Foster Lane as shown in the photo.
The wikipedia page is here