"Lanercost" was written by Maddy Prior (who sings the lead vocals on the piece) and Rick Kemp.
Towards the end of his life, the King spent time at Lanercost Priory, directing a campaign against Robert the Bruce.
Each of the four verses begins by describing the actions of the canons (priests) at the priory - fishing; praying; working in the scriptorium and finally carrying the King's coffin.
The rest of each verse then discusses Edward and his actions - lying in bed "cursing fate" (as he is too ill to fight); delighting at his capture and execution of the Bruce's brothers; leaving to ride north and then dying before his work can be finished.
The chorus is centered on a repetition of the Kyrie eleison (in fact spelled 'elaison' in the sleeve notes[1]), presumably suggesting that this would have been sung by the canons themselves.