Don't know whether you've got a JAWEL paint shop by you but I got some plastic bumper repair stuff,stopper,and primer specifically for this application it's got a flex built in somehow,I used it on a big crack on my Merc bumper and it was really good and hasn't moved,I bridged the gap,which was a good 3/4 inch with P40 and after grinding this down I used the plastic repair stuff and it was easy to use and flat and after priming it you could hardly see it.Even if you haven't got a JAWEL I w ould have thought any good body shop will sell this or some thing similar.
never look down on anyone unless you're helping them up.