Not sure if it helps but I do have a static ip address, paid for, so are you saying that this actually seems to "normally" give a better routing. Funny enough until three weeks ago i had same as you - un-contended beautifully consistent connection. Hence not moving.
I wonder if turning off modem could work - happy to do that - will have to do that over night i guess. For fear of reprisal from rest of household.

Strangely enough I only sent the good WinMtr result to Origin who didn't want anything and the response was "Yes we have raised this!" - and when I asked who to she said "the supplier

So that seems to fit in with your ideas that by seeing the routing they have identified "the hop" that is causing the problem. And I am unlucky to be routing through it.
Thanks again for all that info - so very interesting...