This rendition can be found on the Tallis Scholars' Silver Jubilee Album. Here is a translation of the Latin text: "In the midst of life we are in death; of whom may we seek help but you, O Lord, who for our offences are justly displeased? Yet, O God most holy, O holy and mighty, O holy and mer...
This is actually from the Renaissance period, not the Medieval period, FYI. John Sheppard, the composer, lived between 1515 and 1558.