They seem to have copied the note on the origins of the melody from Wolfgang's posting above (26 Sep 97). Follow up posting indicate that there isn't much similarity between Piva Piva and the tune in the Pathetique.

And to quote Bill Caddick from the "Rough Music" sleevenotes:
The tune is loosely based on a theme from Tchaikovsky's "Pathetique" while the character is mentioned in "Hamlez"