The actual amount of money controlled by the City of London Corporation isn't public either - e.g. there is the "City Cash" fund.
http://en.wikipedia.org/wiki/City_of_London_Corporation
My favourite bit of City trivia is that they have an official representative at Parliament, who gets to sit opposite the speaker - the wonderfully named Remembrancer.
[Edit: For some highly entertaining, if slightly paranoid, ranting about the City and the evils of offshore finance I can recommend "Treasure Islands" by Nicholas Shaxson - http://treasureislands.org/]