"Part of the Union" was indeed written as an anti-union song; it's fairly obvious from the text.  Many years ago, I heard an interview (radio, I think) with one of the writers, who described -if I recall correctly- a holiday job he'd had as a student; he'd been obliged to join the Union that had negotiating rights at that particular place, and resented it -he was only a kid at the time, after all, and didn't really understand what was involved.  The way The Strawbs performed the song on "Top of the Pops" at the time was not exactly subtle.  A year or two later, the song was played at a fairly important Union conference, and I cringed; since then, though, it does seem to have been adopted by people who have either missed the point entirely or who feel that it's a song with a very singable chorus that can be usefully subverted to make a more appropriate point.

Malcolm