You obviously need to look in the neighbours loft as well, and see how bad their side is. The granite blocks look as though a bit of mortar wouldn't go amiss, and I'd then start removing the bricks, and cementing them back in as I go. Looking at that lot, they've attempted supported the purlins, but haven't done a great job, so I'd do the support properly, then as long as you go carefully and methodically, I doubt that it can get much worse than it is.