All seemed to be going okay (I'd cleared browsing history, cookies, plus switched browser) but for some reason the download seemed to fail each time the screen turned itself off. I eventually fixed this so that it wouldn't turn off, or go into screensaver, or sleep mode... and then
this:
Okay I'm trying one more time, but with another browser and from a different source. I've restored the screensaver as this doesn't appear to be the culprit and I don't want to leave the screen turned on. The screen switching off may still be to blame, however, so that's permanently 'on' for now.