Either the killer or police
We can all agree its not a natural position for a dying man to take
Laws of gravity and common sense say he would fall to the floor
Agree with that. Unless an expert says Nevill could have fallen into that position and died. Which has never happened.
This rules out Sheila. She was not strong enough to lift Nevill. She did not have time & would not consider burning his back.
It also rules out the police. They had no reason to move Nevill. It was not police protocol. No officers have ever said they moved Nevill.
The only person left is Bamber. Who had the time, strength and two reasons to.