Not the easiest of nuts to get to. All I can suggest is to borrow enough socket extensions to bring you back through the starting handle hole and, with a T bar on the end, give it a good whack. There should be enough resistance from the engine compression to allow you to do this. Although Martins suggestion is the best way of dealing with the problem if you don't have access to an impact gun this may be worth a go. I have even heard of someone jamming the end of the T bar and flicking the engine over on a particularly stubborn nut, not recommended.