Cancel that guys - worked out the issue - was noted in an older post in here that my buddy found - my problem was using the default Zip program in Windows 10 to do the extracting of the zip files to the SD card
So I downloaded the latest 7Zip (a 2019 version) - used that to unzip the Version 1.04 - stuck the X3 back in the GB and BOOM!! Worked absolutely perfectly.
Might be something to let purchasers of the GB X3 know about as they may come across the same issues as me with newer versions of windows.
They'll get the same error telling them to get GBCOS.zip from the site - but that no longer exists & they should be pointed to
http://krikzz.com/pub/support/everdrive-gb/x-series/OS/and told to grab what ever the latest version of GBC-OS-v1.##.zip in there
Already had a crack on 2 games that I'd never heard of before - A-Mazing Tater and After Burst - so much more exploring to do!
Going to be using this thing heavily during my holiday coming up in a few days =D