I've spent the last 2-3 weeks isolating the metal clunking sound from the rear as well as another thud. The problems were:

1. Rear lower tailgate hatch is a little lose as the rubber has got a bit thinner with age. All I did was make it a bit tighter by loosening the bolts on the black latches attached to the rear wings and pulling them in by 1mm. It stopped the metallic thud sound.

2. The rear seats clasps were vibrating against the plastic of the rear seat. All I did there was put on some pvc tape round the clasp so it connected into the rear seat with less movement. This stopped the sound.

Thanks to Paul, who sat in the back of the trunk whilst we rode around trying to find the sounds.