Just seen there's a Naxos disk of Adam de la Halle's Le Jeu de Robin et de Marion - not quite a folk opera, more of a medieval pastouralle, but appealingly folksy nevertheless. The melody we used for Child #102 : The Birth of Robin Hood on John Barleycorn Reborn comes from Le Jeu de Robin et de Marion, though of course it doesn't concern Robin Hood per-se. An appealing instrumental line-up too: bells, drum, fiddle, gittern/citole, harp, pipe and tabor, rebec, shawm, symphony, copy of Whitecastle pipe, coconuts, tambourine, bagpipe drone, copy of Billingsgate trumpet, cowhorn, portative organ... Just the thing I would imagine for a sunny summer romp in the greenwood.   

Check it out at Amazon