This classic comedy is on an extended run at BFI Southbank. Details here.
Chicago Reader review:
In many ways, the ultimate Billy Wilder film (1959), replete with breathless pacing, transvestite humor, and unflinching cynicism. Most of it is hilarious, but there is something disquieting in the way Wilder dances around his sexual theme—the film never really says what it's about, which might be just as well. Jack Lemmon and Tony Curtis are the two musicians who disguise themselves as members of an all-girl orchestra in order to escape from gangster George Raft; Marilyn Monroe is the band's star, um, vocalist. With Pat O'Brien, Nehemiah Persoff, and Joe E. Brown, who gets the famous punch line.
Here (and above) is the trailer.