From my Witter towbar instructions......
Remove boot floor mat and side storage boxes, remove the plastic nut inside each wheel arch
beneath light clusters, remove the rear plastic panel and the plastic grommets to reveal 1 nut each side, remove nut
taking care not to drop it inside, remove the 2 torx head fixings securing the luggage strap retainers.
Remove the fixings along the lower edge of bumper and the screws around each wheel arch.
As you pull it off you need to flex the wheelarch part outwards a little.
Its a 2 man job, before you start, set out a cloth to put it on.... or put it on the lawn.