'apt-get install bitcoin-qt' simply tells me that it is unable to locate the package bitcoin-qt.
Weirdly, I did manage to get bitcoind with 'apt-get install bitcoind' successfully.
Oops. It turns out that after adding the bitcoin ppa repository thingy, but before installing bitcoin-qt, you have to run 'apt-get update'!
Actually, since all I want to do is download blockchain files, if I can do that with bitcoind I'd be grateful if someone could tell me exactly what to put on the command line. Also I have a 1.1GB blk0001.dat file (and accompanying index file) from mid-2012, so how could I successfully explain to bitcoind that I want it to start from that (saves some downloading).