Nathan was going to kill his brother.

After being pushed into the fireplace by his brother and then being floo'ed to the Leaky Cauldron when he was still lying down he was thrown out of the fireplace and landed of his back. He stared at the roof for a few seconds, trying to get his bearings. After the world stopped spinning and he didn't feel like he was going to be sick, he scrambled to his feet and away from the fireplaces. He rubbed his head where it had connected with the fireplace and the floor. That was going to leave a mark.