rear wheel arch insert

For some reason the bottom section of mine by the rear drivers door has snapped off and it now needs replacing and before I go to the dealer and ask how much it is to replace it I thought I would see if I can do it?

The other thought is to see if I can persuade the dealer to replace it under warranty, which I some how do not think, will happen!

Any thoughts would be appreciated as to an alternative source to get the plastic wheel arch insert