I may well be wrong here, but doesn't it say in the blurb that the equipment belongs to sky, and must be surrendered on request etc etc. If this is so, why would you, the customer have to pay when their equipment has become faulty? You don't pay for TV repairs, if you are foolish enough to rent one.
I know this seems incompatible with the fact that you can purchase sky equipment insurance, but this seems should really only cover negligence / accident on your part. If the box simply stops working, surely it should be down to sky to replace their equipment?