This Fritz Lang classic is on an extended run from Sep 5th at BFI Southbank (and also screens at the Rio Cinema for a week from that date). Full details here.
Chicago Reader review:
Peter Lorre stars in Fritz Lang's sympathetic and terrifying story of a child murderer, filmed in Germany in 1931. The underworld joins forces with the police in tracking down Lorre's plump, helpless maniac because his atrocities have interrupted the course of crime-as-usual. The moral issues are complex and deftly handled: Lorre is at once entirely innocent and absolutely evil. Lang's detached, modified expressionist style gives the action a plastic beauty: the geometry of the images is reflected in the geometry of the plot, as every piece of film clicks together on its way to the inevitable climax. Two lines meet, and Lorre is at the center.
Here is the trailer.