In the Bleak Midwinter is a setting of a poem by Christina Rosetti (1830-94). The music is by Gustav Holst (1874-1934), who is best known as the composer of the orchestral suite The Planets. The sheet music can be downloaded below (underneath the video) or via the Sheet Music page.
This arrangement stays fairly close to Holst’s original setting for 4-part choir. A light touch on the sustaining pedal may be helpful in places, but be careful not to overdo the pedalling.