[Update]: Old friend and retired blogger Francis (Xavier+) Mooney gets it: Franz Joseph (Papa) Haydn's Lord Nelson Mass.
This one's a little tougher. If the composer's genius was lesser than Mozart's, it warn't by much. I need his or her name, BUT ALSO the name of the work (a Mass) from which this Sanctus was taken. (Latin lyrics below the embed.) I've included the Kyrie below the lyrics as a bonus. We used to sing it in choir, and 'twas a revelation. Oh, one additional clue: it is notable for being the only Mass by this composer written in a minor key. The recording's not of the greatest quality, so turn up the sound. As usual, researchligheit ist verboten.
Sanctus, Sanctus, Dominus Deus Sabaoth,
Pleni sunt coeli et terra,
Gloria, gloria tua
Hosanna in excelsis, in excelsis, etc.