ROYAL BALLET RETURN: The two-week Frederick Ashton Cen tenary Celebration at the Metropolitan Opera House, the centerpiece of the Lincoln Center Festival 2004, continues tonight and tomorrow night with the Birmingham Royal Ballet giving the first performances of Ashton’s “The Two Pigeons” to be seen in New York since a handful of them in 1963. On Tuesday, this festival within a festival gathers pace with the return of the Royal Ballet from the London’s Royal Opera House, Covent Garden. This will be the troupe’s first visit to New York since 1997, and the first under its new artistic di rector, Monica Mason. There are basically two programs, the first consisting of one of Ashton greatest works, “Scenes de Ballet,” “Marguerite and Armand” (with Sylvie Guillem in the role Ashton made for Margot Fonteyn) and a varying bunch of Divertissements. The final three performances – July 16 and 17 – are given over to the full-evening “Cinderella,” (look out especially for Friday’s cast of Alina Cojacuro and Johan Kobborg) in a somewhat controversial production (the ballet itself is magnificent, it’s the staging that is controver sial) that was new at the end of last year. Metropolitan Opera House, Lincoln Center, 65th Street and Broadway; (212) 362-6000.