There's a version of it here, in the Digital Tradition, entitled "Why Paddy's Not At Work (Excuse Note)." The words are a little different from how I learned it out of "Rise Up Singing," which I don't have handy just now.
It's fun, easy, and free to use the DigiTrad Lyrics Search option at the top of this page to do, well, obviously...lyrics searches. Although a search for "sick note" wouldn't have turned it up, in this particular case.
ChrisLyrics sent by e-mail, but the address was no good.