BSA B1 in Movies
CLP 821 = mid-36.
Some info here - http://hermajestysthunder.blogspot.co.nz/2010/02/gpo-british-post-office-messengers.html - on GPO bikes but even though there is pic of a similar bike it is not identified. Even though I can't find specific reference to it I think this bike is a BSA B1 - 250 cc side valve - http://www.bsa1930s.nl/1936.htm. From 1937 it was a B20.