Flanders & Swann - don't know the actual title - but it starts "I found a French horn and I started to play it, I found a french horn in a second hand shop. I found a french horn and I started to play it - in spite of the neighbors who begged me to stop." So they finally spirit away the horn - and he take up the Tuba instead! Blat! Blat!

Some time later I found that the tune he's singing to is indeed the e flat Horn concerto (and I find myself singing along to the classical station which is a hoot).