DjerbaDevil, thanks for highlighting that clause, as chance would have it I've just had a new contract sent through this week and the bugger's have slipped in that very clause! Like many of us, I could go direct with an airline but choose not to because I want the agency to take the risk, that's why they take a percentage of my pay. If they're not taking the risk, what's the point in using them?
As a contractor, try and negotiate a weekly pay instead of monthly, at least your exposure will be limited. Good luck!