As I understand from the LP jacket, the song was based on the story of the journey of the magic horn prepared by Morgan le Fay, King Arthur's sorceress half-sister. The horn could not be drunk from by an adulteress without spilling wine therefrom. She sent one of her knights with the horn to Camelot where she hoped to lay a trap for Queen Guinnevere (cheating with Sir Lancelot), but the horn was side-tracked by Sir Lamarok to King Mark's Cornish court where it was drunk from by Queen Isolde (who had been two-timing with Sir Tristram) and all the other ladies at court (few passed the test). - Claire Bear 16 Aug 04

Some relationship presumably to The Boy & The Mantle, Child 29, in which, as synopsised in Wikipedia,

"A boy comes to King Arthur's court with an enchanted mantle that can not be worn by an unfaithful wife. Guinevere dons it, and so does every other lady in the court; only one can wear it, and only after she confesses to kissing her husband before their marriage. Other boys also bring a wild boar, that can not be cut by a cuckold's knife, and a cup that a cuckold can not drink from without spilling it, and these also reveal that every wife at court has been unfaithful."

(This ballad was supposedly sung by Mrs Durbeyfield to the infant Tess in Hardy's "Tess of the D'Urbervilles" - symbolically appropriate but highly unlikely, as it was probably never a sung ballad:- if interested in this aspect, see my article "'Traditional' Lullabies in Victorian Fiction" on p 319 of Notes & Queries (OUP) Sep 1988.)

~M~