Being a trumpet player myself, I am well aware of the greatness of Jeremiah Clarke's Trumpet Voluntary, also known as the Prince of Denmark's March. I remember my dad playing this voluntary for several weddings. I always found it tolerable as a child (which, coming from me at that time, was a good thing when it came to the classics). It is an easy piece to extend, making it ideal for wedding processions.
Its form is ABACA, making it a five-part rondo. It consists of a main major theme in the A section, which is then varied upon in the B section. However, the C section does not have much in common with the A section at all. Looking at the chord analysis, I would theorize that B is a mixture of A and C. The chord progression in A consists of a new chord for every one or two beats. C consists of very long-held chords, spanning over three or four measures at a time. The B section is a mixture of A and C.
These sections can be divided into sub-sections because of the repetition. If we allow a variation of the letters to consist with a variation in instrumentation (i.e. the absence of the trumpet), then the form of the piece is AaBbAaCcaA. The last two A's are switched around most likely to create a more triumphant and solid ending. ("A" consists of the trumpet and the organ whilst "a" consists of only the organ.)
This was fun. Let's do it again sometime.