A sanctus is a hymn for public worship. This particular sanctus is credited to Roy Henry, ie Henry V.
As the historian David Starkey pointed out in his 2013 BBC Television documentary series Music And Monarchy
, Henry V was very much a hands-on monarch who believed in leading from the front, on the battlefield and in every other aspect of his royal life. Starkey said of this composition that it is "musically competent, spiritually impeccable."
Although Henry wrote his music to please God, the Man Upstairs does not appear to have been impressed, and he suffered a fate most unbefitting for a king, contracting dysentery on his final French campaign and dying near Paris on August 31, 1422 aged just 35.
Alexander Baron - London, England