For true believers, Mike Scott will always be the man who invented and defined 1980s stadium rock, and then bailed out just as U2 and Simple Minds were hitting their stride. He immersed himself in Irish folk music, releasing the rollicking Fisherman’s Blues and Room to Roam albums to huge acclaim. Since then he has recorded and toured often, under his own name and the band’s. He also set 20 of WB Yeats’s poems to music for a theatrical presentation called An Appointment With Mr. Yeats, so expect a broad range of very different musical ideas at what has to be their seventh Glastonbury, unless you count the three performances in 2007 separately, in which case it’s their ninth appearance at Worthy Farm.