Well there was a whole load of music played on the river pageant - nine different bands / orchestras on various boats, multiplied by about 4 hours of music, equals one whole bunch of music (in approximate measurements).

If you can find a YouTube / iPlayer of the relevant bit of coverage of the Jubilee so we can hear the theme for ourselves, you'll probably get someone identify it pretty quickly.

As it is I'm sure I'm not the only one who is genuinely willing to help, but you've not given us a great deal to go on ...