Many thanks to those who have replied, mostly off-list, to my question.
The solution probably closest to what I was after (and not requiring the addition of any extra divs) was sent to me by Andrew Krespanis, who reminded me of the old "left:50%; (which will be calculated as 50% of the parent) margin-left:-50%; (which should be calculated from the menu) trick. This works on an absolutely positioned item, whereas "margin: 0 auto;" does not. (Still don't know why.)
Other solutions involved relying solely on "text-align: center;", wrapping the navigation div in an absolutely positioned div, and wrapping the header and navigation div in another div.
Again, thanks to all. So many ways to skin the proverbial cat, but I needed help to see them.
Now for the sub-menu...
-Hugh Todd
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