Nevill's body was in rigor mortis when the photograph was taken. I have tested the position and can confirm that what Myster says is correct. You just can't maintain that position, even with a great deal of effort, so how could a dead body stay like that unless it was in rigor?

One telling detail is that there is no apparent support for the knees. They are bent just as they would be if the person had been sitting before the body was moved and tipped forward. But they just stay up there above the floor. The reason is that the body is in rigor and must have been in that condition before it was put there.
In answer to the above posts I would like to emphasize something which I might not have made clear.
I don't mean to suggest that the scene in the photograph came about entirely by accident. I believe that Nevill was dead sitting on a chair backwards and that when the chair was knocked over, the police deliberately moved his body to put his head in the coal bucket to catch blood leaking from his wounds. But I don't claim to know where the chair he was sitting on was originally located.
The overturned chairs as they appear in the photograph are no real indication of how things were prior to the mishap. Only the cops at the scene know what they found and what changes they made to the crime scene.
These are indications that Nevill died sitting on a chair backwards:
1 His knees are wide apart.
2 His arms are straight.
3 His shoulders are hunched and his head hanging forward, as they would be if his arms and head were hanging over the back.
What happened is that the stiff body remained in the same seated attitude after being tipped forward. It appears to defy gravity, but not if we take into account the presence of rigor mortis.