After a lot of frustration with attempting to clone / expand a .vdi file for Oracle VM VirtualBox I found a CloneVDI GUI tool on the forums which simply does the job.
Hoping this saves others the annoyance of trying the command line utilities which simply do not work. UUID values for hdds don’t seem to be recognised and using named files generates errors of the type “Cannot register hard drive as already registered” which is not the intent of using the “VBoxManage clonehd” command, many thanks to the author of the tool which got me around the problem :)
Why Oracle didn’t have a simple way of doing this type of operation in the existing GUI I don’t know… guess it’ll teach me to ensure that I take account for database data when setting the original max size :P
Edit: The partition will need to be extended (with something like gparted) to use the now unallocated space within the VM, this may involve removing your swap space which will mean that it will need resetting to avoid a major hit on performance.