Yeah, aaronmorris is correct. The filler would expand like teenagers spot and eventually fall out. When you put extended wheel arches on something like an escort MkII you first pop rivet the arches onto the body, then you glass fibre them on, then you drill out the pop rivets and glass fibre over that. If you don't, the rivets eventually pop out of there.
You could try something like Sugru or Sikaflex perhaps, but it probably wont last.
Here at LandyZone we have plenty of very knowledgable members so if you have any questions about your Land Rover or just want to connect with other Landy owners, you're in the right place.