Hi Colin
After some searching I found a thread in the bitnami forum which mentioned something similar. I tried these steps on the relevant folders and succeeded in getting the plugins installed:
sudo chown -R bitnami:daemon /opt/bitnami/apps/moodle/htdocs/theme
sudo chmod -R g+w /opt/bitnami/apps/moodle/htdocs/theme
The question I have now is whether or not there is some underlying ownership issue that may exist for all folders in the upgraded code copy?
Thanks