The installer file was from going via system pref then software update.Where did you get this OS from & how? Recommended: go to System Preferences and then Software update.
Delete what you have and run the above.
based on application folder the file is 8.24gb and called "Install macOS Catalina"What exactly is the name of the installer that you downloaded?
I created a bootable USB and managed to get it working - Thanks
This is why you should have a back up on a separate drive.FYI for others - doing this method erased all data on my hard drive.