Well here was my ziptie fix
As you can see, it's a very small ziptie. All I'm trying to do is stop the front part of that clip from flexing out, there is a bolt on that corner of the bumper that is supposed to hold it on but because the clip can't stay put it just keeps popping out. Because it wasn't actually screwed to that metal bracket below it either there was essentially nothing holding that side of the bumper in place until the plastic clips under the tail light and the rear bumper stay, so any good breeze could have ripped that corner completely off. If my ziptie fix doesn't hold then I'll look at reinforcing that corner with some fibreglass, but I get the feeling that the clip in question is mostly cosmetic. What pisses me off is that a bodyworks shop would have been tasked with fixing this when the damage was originally done (before I bought the car) and a different shop had another go at it when I had the rear bar resprayed a few years ago.
We do have a different scale for tensile strength than you guys too, looks like our Class 10.9 is the same as SAE grade 8 and can hold over 1000 Mpa before fracturing. I was going to go with 316 stainless but that can only take 700 Mpa, so I guess I need something a bit more high-tensile.