Ah, now in my case I hadn't powered it down on my last attempt - tried just now and also included the --reboot command as suggested.

It still didn't work, but I was still using sudo rather than /actually becoming root/ using su.

As far as I know, there isn't a root account on my mac (you can enable it I think, but I'll have to go and figure out how to do that).

I'll give this a try and report back, but it's looking like this stuff doesn't work with sudo (or perhaps there's something else wrong.. :|)

Thanks for all the suggestions so far anyhow!