Well, you pretty much nailed it: It's called "Watercress-O" and appeared on So Cheerfully Round (1967), reissued on CD in 1999 with the eponymous album The Young Tradition (1966). The song was written by Roger Watson, a friend of theirs; he based in on a memory of his grandmother's from a period of privation in her childhood years. It's in the DigiTrad (click) and there's a thread on it: Lyr Req: Watercress-O (Roger Watson). Reinhard Zierke's entry provides the YT liner notes and lyrics.