Byrd left the service of the Chapel Royal and settled in Essex near an aristocratic Catholic family at mid-life. He produced three Masses and the Gradualia, a complete setting of the Proper of the Mass for twelve major feasts and several votive Masses, apparently for clandestine services in this household. This means that the performances were done by members of the extended household (probably quite accomplished), but not by such a great choir as the present Westminster Cathedral Choir, which often sings the music of Byrd--they sang the five-part Mass for the Mass celebrated by Pope Benedict on his visit to England.
Tallis is a somewhat different question, and, as I understand it, his religious loyalties are not as clearly understood. Kerry McCarthy, who wrote a splendid book on Byrd, is at work on one on Tallis, and this should provide an interesting view.

Dr. Mahrt,

What would you make of the claim that the evidence of Tallis' Catholic faith is in his compositional technique, since his Protestant confreres aimed for much simpler styles, in keeping with their Puritanical taste? (For example, even in a piece such as If Ye Love Me he uses a more Catholic style at the end than Protestant music would ordinarily allow.)

Thanks for posting Episode 3, for it's superb.

Indeed, these episodes are also very good.