Can we count The Chavs? Pretty sure they headlined Lost Vagueness in 2007 which was unannounced. I was there with about 30 other people. It was Tim Burgess and Car Barat with possibly Klaxons and one of Razorlight?
EDIT - Checked Wiki
The band appeared in almost full format at the 2007 Glastonbury Festival in Lost Vagueness; with Carl, Jamie and Tim. The band was missing Martin Duffy and Andy Burrows but were assisted by Anthony Rossomando, Didz Hammond and Gary Powell in their place. The band appeared on an open mic stage and turned up wearing fancy dress; only revealing themselves after one song.