[davidimurray]

It's ok - we haven't seen the sun in the UK since 1986!

The IVA manual says -

Rearmost Vertical plane  Where the rearmost designated seat is forward facing; the area starts forward from a theoretical vertical plane 250mm to the rear of the front of the seat backrest (measured where the backrest meets the cushion)

Where the rearmost designated seat is side, rear or centre facing; the area starts forward from a theoretical vertical plane 250mm to the rear of the rear most part of the seat
Where there is a fixed bulkhead or similar within the 250mm measurement this is taken to be the rearmost boundary and is subject to the requirements
Horizontal Plane

The theoretical horizontal plane is determined to be150mm above the top of the lowest seat cushion (measured at the lowest point) extending forward from the rearmost part of the seat back and;

Extending rearwards level with the top of the seat back excluding the head restraint until the horizontal plane intersects the vertical plane. See annex 2

You would need to check with the diagram in section 12 but it would suggest the area behind the seat is exempt