Once home I was able to survey the damage. This is what it looked like.
If you look closely enough, you can see that the rivnuts have pulled out and some of the aluminium has broken away. On closer inspection, I could see that when the rivnuts were fitted, they only secured themselves to the aluminium body panel that was resting on the chassis rail and not in to the chassis rail itself. This wasn't strong enough to take the abuse that an exhaust bracket is likely to give it.
In the picture below you can see one rivnut still attached to some body work!
The resolution to this was to cut away the excess body work to fully reveal the chassis rail underneath. I need to get and fit 2 new M8 rivnuts and then bolt it all back together.
So the moral of this story is to make sure that the rivnuts are clamped up against something solid and not just body work!
Something I had had sat in the garage since collecting the car was the trim piece that runs along the top of the rear panel and hides the boot cover edge and the edge of the carpet. I got this from GBS and thought it was an aluminium one and was going to cover it in either leatherette or carbon wrap. When I took the plastic wrap off it, I found it was a stainless steel, so I decided to leave it as it was. It is simply bonded in place at either end with Sikaflex.
Here is the finished job with the harnesses fitted back on.
I also went round the car and did a spanner check. I have done 250 miles now so it needs checking over. There were no major finds, in fact most nuts and bolts were still as tight as the day they were fitted. Even the nut in the driving seat!!!