This is episode three of Simon Schama's impartial, detailed and politically incorrect A History of Britain. If you skip to about 43:30 you can hear what I think is Emma Kirkby. Does anybody know what she is singing? If you can ignore Schama's commentary on the feud between Henry II and Richard Coeur de Lion it's rather beautiful.
One thing I found amusing about this part of the documentary was Schama's treatment of the manner of King Richard's death upon the battlements of Châlus as, "a triumph of daredevil romance over common sense."