Thanks for the reply, 'atanere' but I've solved it with the command
resize2fs /dev/fedora_localhost-live/root from the terminal pane/window. Essentially as in my reply to the "Wizard from Oz", /opt is 'hiding' inside /root, thanks again for the input.
Cracked it 'wfo' aka Chris using:
resize2fs /dev/fedora_localhost-live/root
essentially /opt is 'hiding' inside /root! See attached 'png' file showing there will now be enough space, magic.
Thanks 'woo' the last thing I tried was running the "gnome partition editor" (gparted) from disc on cdrom and running terminal commands from there, but unfortunately no difference: haven't given up quite yet!
Thanks for your reply 'WOO' (Wizard of Oz) I've decided to use "VBoxManage" in the following way:
VboxManage modifyhd “Fedora 27.vdi” - -resize 150000
To increase the overall size of the vdi to 150GB; awkward because it means I'm scraping the barrel with the host machine of 750GB capacity i.e...
I need 27.32GB to install an application in /opt but disc space available is 8.57: a bit of a discrepancy! Unfortunately (?) Fedora 28 uses lvm2 so 'gparted' isn't much good. NB: Fedora 28 is set up in a VirtualBox within an iMac host running macOS Sierra with the following 'virtual size'...