On the outside edges of the third row there are black tabs that you can pull. 

 

They don't appear to do anything but surely they have a purpose. Presumably to remove or tilt up the seats? 

 

Does anyone know?

Mystery solved. Pull both at the same time and they fold up far enough to remove the spare wheel. 

There you go. Happy motoring.

Yep, they unlock the small gas strut that tilts the folded third row up and forward to gain better access to the spare wheel.