It's a bumper off job though, or it was on mine at least to remove that black plastic mount. They are riveted and screwed on from the inside of the bumper. It's alot easier to carefully remove the number plate, put some card down the back of the black plastic and then spray it carefully in situ. Get some new double sided pads and stick the number plate back on.
A tip I pick up is to go over the faded black plastic with a heat gun (One used for paint stripping) It brings the black back out and then all you have to do is use your normal cleaning product. I used this technique on my Land Rover and it worked a treat.