Hello Bjorn and any other interested members,  The day after your
article appeared, they happened to show this film on BBC TV, so
I was able to study the scene to which you refer.

Sure enough, it starts with an 89-key organ of some description
playing quite distinctly in the background.  However, later on, as
Arthur Mullard's hair is being well and truly burned, the recording
definitely changes to the Becquart Hooghuys playing the "Blue Devils"
march.

10 points for observation!

Rob Barker, Lincolnshire, UK, where the sun never rises.