Thanks due for all the help I've received from Benjamin Mako Hill and others. This post should now arrive on Planet Debian.
Still struggling with a Marvell Sheevaplug: I've now got it to (not quite but Oh so nearly) boot: if you remove the SD card it whines - and recognises the partition layout - but doesn't boot from it, hanging on a "waiting for boot prompt"
The next trick is to try an install to a 4G USB stick and see if that works, then try and get new/different SD cards to try. WARNING - the supplied USB cable for serial console is significantly short and weak :)