RPI4 USD SSD boot issues

Okay, so what's the knack of getting USB booting from SSD to work with arm64 20.04 desktop on an rpi4? No matter what approach I try it seems to hang during start up. The boot screen with the rpi logo & QR code flashes up and then just a blank screen ever after. It all runs fine from the SD card.

I've tried these instructions: (Optional) Setting up USB Boot and had no joy.

Cloning the SD card with the dd below didn't make any noticeable difference:
sudo dd if=/dev/mmcblk0 of=/dev/sda bs=2048 conv=sync,notrunc,noerror status=progress
Nor did copying the SD card to SSD using the rpi SD Card Copier app.

64bit Raspberry Pi OS works fine from the SD card and also boots fine from the same SSD when cloned using the dd above, and also if the SSD is prepared using the SD Card Copier app.

There are a couple of other posts on here that I've been through and they don't seem to resolve things either:

Any pointers gratefully received!