The song is White Dress, from the album Rising for the Moon. The song was written by Dave Swarbrick for Sandy.

Over on YouTube there are a bunch of people saying it's not Fairport, it's Fotheringay. Well, it's Fairport: Sandy, Swarb (mandolin), Jerry Donahue (electric guitar), Trevor Lucas (acoustic guitar), Dave Pegg (bass), Bruce Rowland (drums). Some were also saying it's Richard Thompson on guitar and that Ralph McTell wrote it: no and no!