I learned this one from the guy who does my body work. If your big front bumper is not sitting tight against the wheel arch trim, you will see that there is a small plastic finger on the bumper that should slot into a hole in the arch trim. The fit here is not always great and the slot and hole need a bit of persuasion to mate together.
Get a mate, if you have one, to push the bumper at each side until the finger slots into the hole, and from the back, slide a small jubilee clip over the finger and tighten it up. This ensures that the arch and bumper remain snug.
Its easily done but effective