Thanks for that diagnosis, auntyneddy.
Have had an annoying rattle in the boot/rear seat area for ages and couldn't find anything obviously loose.
Have just followed your guidance, and these metal plates under the plastic covers on both sides were loose.
Don't think there was any broken plastic. The screws had just worked themselves loose.
Have just tightened everything up again, and will wait to see if rattle has gone.
However, I can easily see them coming loose again, and think it may be worth replacing the self-tappers with nut & bolts (if necessary).