Drill through the flange with a 1,5mm drill bit, place a masonry nail through the flange, such as you find on plastic cable clips, then unscrew the M6 screw.
Now comes the clever part, obtain a 1/4" drive socket where the diameter is approximately, that of the diameter of the flange of the rivnut, and take a 12.9 M6 bolt long enough to pass through the socket and into the rivnut.
With a pair of mole grips, nip the socket so it cannot rotate, and tighten the bolt into the rivnut to 20Nm
Trust me when I say, there is not a pair of rivnut pliers on earth that will nip up as well as this.
When you've done this, liberally apply coppaslip to the threadform of the bolt that secures the lamp assembly.
Brian
