Re-upholstering Hans Olsen shell chair  

slots on to
2 or 4 screw heads with a little force.
Pull the back pad upwards and it will release! should only take a minimum amount of force UNLESS some nimwhit has glued them too.
Lift the edges using a flat blade and you will see the flexibility in the laminate.
Alternative if you are reupholstring just run a stanley blade over the covers and that way find the fixtures once you have stripped the foam back!

Thank you, Simon!
The pad was indeed slotted onto two screw heads in the middle of the back rest.