Purple Dancer & Dragonfly are certainly two of the better songs to come out of this period (and there were plenty). I don't understand why they aren't more readily available.
The fantastic discography on here (fleetwoodmac.net) says that Jeremy provides guitar and vocals to Dragonfly. Certainly Purple Dancer is more of a duet between Danny and Jeremy, each taking alternate verses.
If the discography is correct, then Dragonfly was written by Danny with W.H. Davies (wasn't it a poem by W.H. Davies?), and Purple Dancer was written by Danny, Mick and John.
http://discog.fleetwoodmac.net/songs...59&perfid=4025
http://discog.fleetwoodmac.net/songs...77&perfid=5200