What proportion of heat comes from the back of the fire and the flue in comparison to the front and sides?
This is a difficult question to answer. Most of the radiant heat from the fire comes from the top plate (more front than back) and the door area. The cooler convection heat comes off the extruded coloured panels around the sides and the front of the fire.
By comparison, the heat from the rear of the unit is much less than that from the other sides but it can still get to quite a temperature around that area. What that temperature is can vary depending on what the fire is doing, it is almost never a static output.