The Mermaid Singers and Orchestra

unless it was a house chorus and orchestra, most likely an ad-hoc credit given to the chorus and orchestra that was directed by Geraint Jones (it might even have been Jones' own chorus and orchestra), named after the Mermaid Theatre where the 1951 production of Dido and Aeneas (with Kirsten Flagstad, Maggie Teyte and Thomas Hemsley) was staged, see: https://en.wikipedia.org/wiki/Dido_and_Aeneas#Recordings and https://en.wikipedia.org/wiki/Mermaid_Theatre#History

the 1952 cast recording of Dido and Aeneas (with Elisabeth Schwarzkopf in place of Maggie Teyte as Belinda) evidently retained the use of the credit "The Mermaid Singers and Orchestra" (see 1952 vs. 1951) => https://en.wikipedia.org/wiki/Dido_and_Aeneas_discography