Thanks for fast reply!
Yes, it did arrive unformatted - so first I formatted and initialised with iTunes 7 under Windows, did a manual install of rockbox under Ubuntu, and got this screen. I can't remember exactly what order I tried everything else in, but it includes uninstalling and reinstalling rockbox both from ubuntu and using the automatic installer under windows, reformatting and reinitialising the hard drive using windows explorer and the anapod reinitialiser (wondering if it might be some updated Apple firmware that might be the problem). Tried installing ipodlinux's ipodloader too, but failed because I couldn't find the right files to download. Throughout, ipod boots into the Apple firmware very happily, and keeps getting stuck on the same rockbox bootloader error! Like I say, baffled...