Michael - thanks for your suggestions. Tapping it did not help, and when I used the wrench extender all that happened was the jaws on my 10mm wrench opened up enough to allow the wrench to turn!
For the benefit of the next person to hit this problem, I finally got it off by buying an
extra long ratchet wrench and some
low profile hex drivers. This combination can (just) reach up into that super confined space, and gave me enough leverage to get the filler plug off.
My three wheeler is now happily freshly lubricated, and simply purring along.
