“Alzheimer’s, 9/11, Asbos, the Yorkshire Ripper, the ‘war on terror’, the apocalypse – there’s no shortage of provocative topics on the Edinburgh Fringe this year. The extraordinary thing is they are all being tackled in musicals. It’s striking that song-and-dance extravaganzas can be staged out of such sensitive subjects. How did a ‘low’ artform get so grown-up?”