Stand down folks - this looks like a problem with the IO boards, some of which seem to be dead. I've found one that seems to work reliably now.

Ahoy all, I've noticed some strange behaviour which I wanted to clarify - a "naked" uninitialised pi compute module seems to accept the J4 / "sudo rpiboot" procedure just fine, but it seems that when I try the same on a board that already has something bootable on it that J4/rpiboot doesn't work. Is...