You actually turned out to be correct Mark, seems the newer version is causing issues. I was advised to go back to v6.0 and that fixed the issues.
This was the main issue:
Just incase anyone else has this issue, these are the Linux commands I used:
Remove existing version and its files:
apt purge --autoremove "virtualbox*"
Install a different version:
apt install virtualbox-6.0
I think half the problem is the progress bar and estimated times don't really work atm so you have to ignore these. Average tasks take around 2 hours, however admins advise just to leave tasks running as 99.9% will complete. I can confirm this as I just had one that completed after 16 hours.
Also another issue is the huge amount of data the tasks continuously download while running. 4 hosts maxed out a 40Mb (down)/10Mb (up) connection! In less than 24 hours these hosts have download over 250GB.
Also there is a period of about 5/10 mins per host when it starts up (and starts downloading its essential data) which puts huge load onto the host, so if you start loads of tasks at the same time you just have to wait 30+ mins for the host to not keep freezing (due to trying to do so many concurrent downloads in the background).
If you keep all these issues in mind, and the 650GB RAM requirement per task, then the Theory tasks work fine.