Thanks for the tip. I've been testing Backblaze for the last 24 hours and I've managed to upload 140Gb. This is much faster than my tests of Crashplan, which only managed 25Gb/day.
I have 5Tb to backup, so I'm looking at 1 month with Backblaze vs. 6 months with Crashplan. I'm less worried about the length of time for the initial backup, than whether the backup process will keep up with me when I'm making a lot of large files (e.g. preparing print files for an exhibition).
I'm based in Europe (London), and Backblaze allows multi-threading in its backup process. This apparently speeds things up for those users who are a long distance from the Backblaze data-centre in California - https://www.backblaze.com/blog/backblaze-online-backup-4/
So far, it seems to be working for me. However, time will tell - Backblaze seems to backup smaller files first (text files and jpegs) so it hasn't yet tackled any of my raw files and psds. Perhaps these will bog it down. (Crashplan, more sensibly, gives priority to your most recent files.)
(Connection. BT Infinity. 60 Mbps down, 18 Mbps up.)