Nothing About Nurses!

Sometimes there are things I come across that are nothing to do with nurses at all, but I really want to know more about them.  While I was clearing my late mother's flat, I came across quite a few photos of my father's time with the Royal Sussex Regiment and later the Army Physical Training Corps. With them I found this photo and it's always interested me.  Obviously taken at the Royal Hospital, Chelsea, but I wonder if anyone can date it, or add anything about the men pictured in it - I'm completely clueless about it. A sticker on the back titles it as 'In Full Song.'