Well, I thought I better take the rivnut idea to it's logical conclusion and try it in a back box. Might be of some use to someone else thinking of investing in one. I had a go first on a loose 1G Appleby - they're a pain as in the multiple knockouts in the back leave only really the centre hole as an option for the rivnut. The corner holes - well you can't get the tool in!
The component parts of the rivnut tool alongside an M4 brass screw:
No chance in the corner hole:
But no problem in the middle:
And all "clinched" (I'm loving the word "clinch") up:
So to the problem back box, easier to place the rivnut in this old box as the multiple knockouts in the back are spaced differently. Drilled through the steel back then into the stocks behind with a masonry bit. Removed the dust etc:
Yes, it might have been easier to replace the back box but as I say to the missus, if you don't try these things you'll never know if they work!
No decor was harmed in this process either. Thanks for looking.