Both the dedication to a Roman saint (cf. Canterbury) and the discovery of a C7 altar stone suggest that this was one of the earliest churches to be established in the London area. However, by the Middle Ages the main settlement in the parish had shifted northwards to Kentish Town, and the little building stood alone in its large churchyard until surrounded by expanding London in the early C19. The medieval church consisted of chancel, nave and W tower. Surviving details, much restored, indicate a C12 rebuilding: in the chancel a Norman S doorway with two-way chevron (second picture in upper row), in the nave traces of a Norman N door (visible inside - not shown here). Remains of a C13 lancet in the chancel N wall (not shown here). The church was crudely Normanized in 1848 by Roumieux & Gough, who removed the medieval tower and replaced it with a W extension with W gallery, and a S porch with tower and spire above, truncated later, when the incongruous half-timbering was introduced. ... 
The last picture shows the medieval church in 1800 (a plaque in the churchyard).

Major article

In the churchyard, Sir John Soane's tomb