@Burakkucat - Indeed, I do think my isolated location and first-in-the-village situation should help with my clean low noise environment. I was thinking of things like radio stations, when I used the term "distant".
The amazing high value was only sustained for a couple of days. But on the 14th, many tests were done and all gave spectacularly high d/s or u/s throughput figures. Comparing the 14th with the 12th Dec, downstream IP throughout went up by ~400k as measured repeatedly using two completely different speed testers. So it was not just a glitch in speedof.me.
I took half a dozen measurements in each case, recorded all the numbers, and took the arithmetic mean and the max. (I would have thought that geometric means would be more sensible, but I actually want the bias from the high values.) I'm much more interested in the max as I believe that is reliable, because alien local or remote competing traffic or slowness within the remote end test server itself can reduce the figures, but external disturbances can't improve results. The very lowest outlier results, if any, in each group were discarded, assumed to be a failure due to the network not being quiet. I checked for alien traffic going through the router if any bad results were consistently seen.