Eventually, the front chair toppled over, and with the ever increasing pressure being exerted on the opening edge of the door by firearm officers, the chair which Neville Bamber had been sat upon, also started to topple forward at a slight angle, sending Neville Bambers body crashing forward ending up looking as though his body was balanced somewhat precariously on top of the original forward positioned chair!
I believe that this is the correct interpretation to account for how Neville Bambers body ended up where it did in the crime scene photographs, and just for good measure, cops placed his head into the coal hod!