Well strap me to a tree and call me Brenda!!!!! How nice of MS to tell us something we already know!!!!!

Downloaded a Win 11 iso at just over 4.1Gb which was accepted by a usb drive, but in reality probably just under 4Gb. But although set to boot from usb in the bios, it would not. However a mounted version has now booted, and happily went to the command screen at the install error with shift+f10.

So it. now sits at X:Sources awaiting diskpart.