Great topic. A few that come to mind:

"Beware of Dog" on Kevin Ayers' Banamour

"A Series of Superbly Played Mellotron Codas" on Lol Coxhill's Ear of the Beholder

"The Other Stubbs Effect" on Hatfield and the North's first album (not so much an afterthought as a bookend to the first track, "The Stubbs Effect"

"Slush" on the Bonzo Dog Band's Let's Make Up and Be Friends"

I'll post later on some complexities.