I know it as a Flanders and Swann piece where Flanders set the Mozart E flat Horn Concerto ,(I believe K.495) to music, it starts out
I once had a whim and I had to obey it to buy a French horn in a second hand shop
I polished it up and I started to play it, in spite of the neighbors who begged me to stop

Is this the one? I can more or less sing it, but if someone has the Songbook they could do a better job PETE