By "main bit", are you referring to the header panel? That's the frame above the bumper that the headlights and grille mount to. To straighten out the grille, place it in near boiling water for a minute or so, until the resin is softened. Remove it from the water; and, using two pieces of styrene sheet placed on the bottom of the insert, press both sides down with your fingers and run some cold water to cool the resin. It will flatten and retain the shape. You didn't try inserting the grille into the header panel in its present state, did you? That vertical divider would prevent you from doing so. Grind, or cut out, the divider in the header panel and the insert will fit.