This is very short notice, but...       This is the play Impossible that debuted last summer at the Edinburgh Fringe, with Phill Jupitus and Alan Cox as ACD and Houdini. It's nothing to do with the spectacular magic show of the same name.       The reading tomorrow is led by David Calder and Kerry Shale, with Diana Quick, Jessica Martin, Catriona Cox and John Dorney. It is not open to the public at large, but there are some seats available. If…