Thanks for your interest and offer to highlight the problem through other channels, Mixture. I appreciate that I might be an "edge" case, but by the look of various discussions online, I am far from the only one with the problem.
For now, I will try one of the two "work-rounds" that I have seen suggested, either making sure wifi is off any time I have it connected to mains power, or loading the devices so full that there is no room for the download.
(highflyer - if you read the previous posts, my problem is not with Updates, but with Update DOWNLOADS)