After sleeping on it ....
.... bumper bar shows some crazing at the side - adjacent to a small dent in the rear wheel arch
Guess a parking incident put stress on the bumper bar and its subframe, causing the subframe to break away from the pop rivet fastening it to the bodywork
Assume pop rivet because subframe side still fastened has the inside of a pop rivet protruding into the spare wheel well
So I re-fastened the plastic subframe to the bodywork with a M5 screw, flat washers either side, and a Nyloc nut
Remaining fasteners for the plastic bumper bar to the plastic subframe seem to be those push-through plastic barbed clips
I decided not to use metal fasteners here, on the assumption that original plastic clips are meant to shear or pop out if the bumper bar flexes, rather than bumper bar mounting point breaking away
So I joined them together with a good old cable tie which will likely come apart under stress instead of breaking the bumper bar
Job done
