It's interesting that you are so sure it will fire turning over slowly by hand. Just wondering have you actually tried. I'm not saying it won't, but i have tried both with the plug in the engine and with the plug out of the engine earthing against the engine.
I was doing this trying to determine if i had a faulty ignition box. The only time i have had the plug fire is when turned over with a starter, not by hand. But like i said earlier, it's good practise not to tun by hand with the ignition on.