By grills do you mean the flapped vents? Well there's supposed to be one on each side but for some reason my drivers side was just missing! So I just bought a new one and fitted it.
I approached the task thinking I would have to remove the bottom plastic valance completely and then remove the metal panel on top separately. But I just removed both together still attached to each other.
For future reference to anyone else that needs to remove both, here is how I did it
Remove rear lights.
Jack up car and put on axle stands.
Undo the two bolts with a torx screw under each light (4bolts total)
Remove the four bolts from underneath the plastic valance
Remove three screws from each wheel arch that connects the two panels, and the arch liner.
Pull the whole lot off.
Remove the electrical plug for the number plates light
Voila
You need a t30 (I think)
A t20