Problem sorted - it turns out you need more than a few GB free on the hard disk, before it will patch successfully. I think it was struggling to decompress the files (not that I had a shortage of space, but it seems a 3 Gig just isn't enough for it). I cleaned out some space, and it worked like a charm.
Thanks for your help, and I hope if others stumble across this problem the same solution will work for them.