I managed to get it right by following the guide below:
https://forums.virtualbox.org/viewtopic.php?t=85631
Hope this helps anyone else experiencing the same issue.
Added in 19 minutes 43 seconds:
[ref]MaLd0n[/ref], it was related to the type of file system being used. VirtualBox doesn't yet support AppleFS, so I had to tell the installation to use the recovery partition as that is where the install files get copied to. It then completed the installation once I had done this and ejected the ISO from the VM.