One way this can happen is when an older offline executable is used and requires an internet connection to update to the latest version available. Example: 17.8.3760.0 to 17.8.3760.249 installation update. Changing your DNS can ensure a reliable internet connection and allow this to happen.
Thank you to both Alikhan & mchain for your input. Sorry I haven’t responded until now, I have been busy, and now have tried a few solutions.
Turns out that the reason for the Fatal Error was that the offline install needs you to be online, else it fails with the above error code! Once I ran it whilst connected it completed without a problem, didn’t have to change DNS server or anything.
Makes the name offline a bit of a misnomer. Even so it shouldn’t fail with a fatal error rather than asking to connect.
See reply # 2. Exactly why an internet connection might be needed if using an older offline version installer. Even the latest one might require some updating to some components if emupdates were applied since that version was released.