I had a couple of appointments in London today which took me from leafy Kingston Upon Thames into the City of London and in between appointments I walked through The Temple and across the 'boundary' of City of London/City of Westminster. Have you ever seen the boundary marker for the western-most point of the City of London? It's a silver dragon with arrow tongue and tail standing on a plinth. The dragon symbolises the City of London and bears the red cross of St George featured in the City's coat-of-arms. Apparently, these are dotted around the boundary of the City of London.
The City's boundaries have remained almost constant since the Middle Ages. In the medieval period the City was the full extent of London, and distinct from the nearby, but then separate, settlement of Westminster, which became the City of Westminster. The City is not one of the 32 London boroughs. (Source: http://en.wikipedia.org/wiki/City_of_London)