During an interview many years ago, on David Letterman's show, John Cleese told the story of his first work in America, as a chorus singer on Broadway, as I recall. The point of his story was that he cannot sing, and although he managed to get the job anyway, he more or less pretended to be singing throughout the length of the job.
As a result of having hearing that story, I was quite surprised to hear the following selection, which I've excerpted from an episode of the long running BBC-Radio comedy show, "I'm Sorry I'll Read That Again", which featured Cleese before, throughout and after the run of Monty Python, during its mid-'60's to mid-'70's run.
On the other hand, I was not surprised at the quality of Cleese's singing or of the opening joke, "Anybody should be singing it but John Cleese". I hope you enjoy Mr. Cleese singing his heart out on "The Ferret Song".