Thanks for the suggestions, but the problem is the wrenches I tried, both metric and conventional, don't fit. It appears to be between a 10mm and 11mm, or between a 3/8" and 7/16", and as far as I know, they don't make in-bewteen sizes.

Michael, you must be British... I first learned what a "Spanner" was in Orwell's 1984; it sounds so much less vomitous and painful than "wrench" . Like I said, though, I'll just work with an adjustable.

Thanks!
Joseph