Thank you Chris! You are a legend! I have no idea how you managed to dig these up - I literally spent a day trawling around the various Emagic websites on the wayback machine and came up empty.
Those are indeed the floppy images you need to install Logic Platinum 4.0 from the older Logic CD's (mine is labelled Software Master CD Volume 6).
You have to use Disk Copy to mount the disk image first and then run the installer on the Logic CD. The CD installs the software and then runs an updater on the floppy to activate the application.
I test installed to one of my beige G3's, attached my adb dongle and now have a fully functional Logic Platinum.
Thank you to everyone who chipped in with advice, it is much appreciated. Please upload these disks to the Logic Software section here because they will be important to archive for the future.
Cheers, Simon W.