The song comes near the beginning of the show -- told in flashback, as Pickwick is in prison for refusing to pay for losing a "breach of promise" suit, because he never proposed, but his widow landlady Mrs. Bardell misinterpreted his words. Pickwick reminisces with his valet Sam Weller, who's come to visit, about when they first met. The score is by Leslie Bricusse (who usually wrote with Anthony Newley) and Cyril Ornadel, who did the music.
This is Harry Secombe, Teddy Green and the fellow-debtors singing "That's What I'd Like for Christmas," from the original London cast recording