Live broadcasts into cinemas have become something of a commonplace, and a welcome one: operas, theatre performances, even radio programmes. But a live broadcast into cinemas of the audience entering another cinema is a new one on me.

The idea is part of this year’s BFI London Film Festival, not as an avant-garde experiment but as a way of adding lustre to the closing-night gala: Saving Mr Banks, a sort of biopic of the film Mary Poppins with Emma Thompson as author P.L.