Thanks, guys... I'd turned the switch off option to 'never' but kept the screensaver as this wasn't interrupting the download. And, as suggested by my pal Threadfin, I also dumped my browser history and cache and made the next download using a different browser. Just for good measure I also made the next download from a different site.
I really thought I'd cracked it when I got up this morning and found it was still going and had reached about 1.7G of a 2.0G download. Then, suddenly,
"Failed... server unreachable" I think I've had enough now