Probably filmed in Pathe Studios, London.
Shots of musician Musaire playing a version of a Theremin - The instrument has two aerials and by moving his hands Musaire changes the pitch and volume of the notes. He plays the tune 'Miracles Sometimes Happen'.
He then explains a little about his instrument. He does musical impressions of boat horns and seagulls. He then plays 'The Fleet's In Port Again'.