My update has just finished! Took around 2 hours to do step 1 "preparing for update" but then only a minute or so to do the step 2. Thank goodness. Glad I didn't interrupt the process. I had the luxury of it being midnight and I could let it go for awhile to see what happened.
Surprise, surprise! It warns not to unplug - bunk (in this case). A quick (15 second) unplug and it restarted, ran through the portion that was already downloaded very quickly, then carried on from there. Looking at the What's New screen now. Success!