An update.
I've just had an email from the couple. They've run 2 speed tests using the instructions I left with them last June

. They show download of 4.54 and 4.46 Mbps; upload 0.62 and 0.61 Mbps. And their ping is 27 and 36. All so much better than last Monday.
I have to say that although the reason for their banding is impossible to determine, the ease with which I could contact Eclipse and get the issue resolved makes me appreciate the benefits of a more expensive ISP. I so rarely need to contact Eclipse - either for myself or others - but when I do it's a good experience.
Tony
Happily running Windows 10 Pro on both desktop and laptop
We have more and more laws, and less and less enforcement