I've never noticed the shed and I pass there regularly so it can't be that big. Don't worry about subsidence its not that sort of mining - you're more likely to fall down a pothole or a mineshaft. In fact, a friend of mine from Litton (just over the hill) - we don't talk about him much 'cos he plays the banjo - found a new one last year; only 700+ feet deep though. You could always pitch your tent a couple of hundred yards up the road - nice flat field - used to be the site of a smallpox hospital.