Well, for that you need to fit it on a 720k floppy :) Unless you modded it with a 1.44 drive

Yes it runs on that Amstrad. A funny story about the Amstrad, at one point we added divide-by-zero trap handler in the kernel for user space apps. When the Amstrad reboots via our 'shutdown', its gets a div zero exception in its own BIOS (which at the time prohibited the reboot, lol).

I'm having a play on the PPC640 with a Gotek running Flashfloppy with fd720-fat.img

It's erroring after the mount with panic:No init or sh found SYSTEM HALTED. BOOT:

The computer is unreponsive at the boot: prompt.

I've tried a few different settings in the ff.cfg, but no joy. Could it be a Gotek issue?

Open up an issue on GitHub and we can help you better. The usual reason for problems like this is the layout of the image is incorrect on GoTek, and/or the GoTek is set to the wrong CHS (cylinders/head/sector) for the image being used.

Also worth trying would be the 1440k image, as that's the later standard 3.5" floppy format.