Got your PM mate.
I just got a rear bumper, chopped out a big section, chopped out the same section on the new bumper then plastic welded it in.
I used a fine tip torch and just melted strips of the plastic in.
DON'T use fibreglass! It just cracks, plastic weld it properly, then decent U-Pol filler try and weld it well enough to use little filler.
I then crashed it.