There are four screws on the underside of the diffuser t25's i think they are.two in the centre and one on each side holding the arch liner.once these are out the diffuser is a little more durable.then comes the hard part.you now need to get up underneath to release the clips holding it in place,you wont be able to just pull it off,you need to get a driver in and price them apart.once there unclipped you can then remove it.also the honeycomb sections and reflectors can be unclipped and removed.
A word of warning though,it was not easy.if i was to do it again i would remove the rear bumper then then diffuser as it would be easier to get at the clips.