I am looking for a Sarum Credo incipit (Latin text) that will fit polyphony with a final of C - Tallis, Mass for four voices. This will also be transposed up a 5th in another edition. I assume that the Sarum melodies may not be substantially different from those in the Roman Rite, but I just want to be sure. Roman Credo III (mode V) seems to fit best, beginning on G. I see that is in the 'St Dunstan Kyrial' for use in English, but it is a 17th c. melody. Might it have been in use earlier? Can someone help, please?