Kiss Me Again by Margaret Moore

Lady Francessa Cecilia Epping has carried a torch for the Honorable Brixton Smythe-Medway for years, until she discovers that Brix has wagered his friends that he won't marry till he's 50 and he will never, ever marry 'mousy little Fanny Epping'. Francessa makes a counter wager: she'll break his heart in six weeks. She then seals the bet with the most mind-boggling French kiss Brix has ever experienced. Who knew Fanny could kiss like that? Who knew she knew how to kiss like that? Good God, wherever did she learn to kiss like that? Poor Brix! Poor Fanny! They're so busy trying to pretend they don't have any feelings for each other except anger, and yet they keep kissing. Again and again and again.