St Andrew Undershaft is a Church of England church in the City of London, the historic nucleus and modern financial centre of London. It is located on St Mary Axe, within the Aldgate ward, and is a rare example of a City church that survived both the Great Fire of London and the Blitz.Wikipedia
Another one that can prove difficult to visit. It has been open during London Open House weekend. Friends of the City Churches recommend you arrange a visit via the office at St Helens:
Telephone: 020 7283 2231
Nearest stations: Liverpool St., Aldgate and Monument.