Semper aliquid

Having prudently done my shopping and gone to the gym yesterday, I had nothing in particular to do today. I started out for the Transport Museum, which I haven't seen since its refit, but there was a long queue outside, and I didn't fancy standing around in the flurries of hail and sleet and almost snow.

But a rather wasted journey offered - as London tends to - a moment of intrigue.

What was on at London Bridge that required this young man to bring a sousaphone?

And why the sunglasses?