Well I'm impressed Stephen, not only have you replied twice very quickly, but you've helped me get a bit further each time. I managed to fix the errors by cd'ing into the cache directory as you said, and as I type this so 'apt-get -u -t unstable dist-upgrade' is doing its stuff :)

I'll report back on how I get on... quick question 'apt-get -u -t unstable dist-upgrade' updates every package on my system right, except of course the kernel and its modules? Tomorrow I'll print out the apt guide (O'Reilly pdf) referred to in the apt-get sticky of the hard rive install forum.