Mark Ravenhill: Yes, I'd work for free
... But I'm not doing it for David Cameron's big societyBroadway musicals aren't necessarily the best place to look for wisdom. There aren't many practical clues to leading your life contained in show tunes. I've never attempted, as The Sound of Music recommends, to climb every mountain until I find my dream. Or waited until the end of the storm (thanks, Carousel) for the sound of the lark. But I have, until recently…