@girish Yes, all sorted, thank you for asking.
Thanks to @robi's suggestion all we needed to do was the following:
in terminal window for our WordPress app, at /app/code/wp-content/uploads we created a symlink (ln -s /media/ovitest ovitest) and then updated its permissions, just in case (chown -R www-data:www-data ovitest)
then we could access the mp4 at https://<our_web_domain>/wp-content/uploads/ovitest/Red-cloud.mp4
Thank you very much all; we did enjoy finding the fix in the end. 🙂 And of course we will make good use of it as we can now keep 10-20 GB of data, or even more, outside the WordPress data container, so out of the backup, but still reference this content on our website, which is great.