How to center header over body text?

L/R margin settings that apply to Headers cannot differ from those applied to the body text. If the Header isn't centering above the text it has to be due to an intentional or accidental layout modification [such as indentation or tabs] applied to one or the other.

Exactly how are you imposing what you describe in your opening paragraph? Are you using the features of Format> Document> Margins or are you making some sort of manual adjustment in the document, itself? If the former, what specific settings are you using?

What is indicated by the Ruler when in the text? When in the Header?

The settings shown here:

automatically provide the result shown here;

Notice that the Header is properly centered above the body text on both the recto & verso page.

I may have just found a workaround - I looked up at the margin at the top, and it was indented, so I grabbed the tab marker it and dragged

I now have centered headers! Thank you.

While we're on - can I ask you how best to set up the document so that there is a non-numbered page at the start?

The point is to have odd page numbers on the right when in two-page view.

I managed it a long time ago but have no idea how I did it. Adding blank pages doesn't work.

Assuming that you aren't dealing with multiple Sections & do not want the Header to appear on the first page either, the simplest method is:

1. Go to Insert> Page Numbers
2. Choose Position & Alignment
3. Clear the check for Show number on first page
4. Click the Format button
5. Enter 0 as the Start at: value

Depending on a number of possible variables in the document construction it may be necessary to take a different approach.

I was going to offer this, but Bob's method seems the quickest.

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If Bob doesn't blast me out of the water, I can think of two other possibilities:

1. Go to the "Layout Tab"  and click click "Section Break Next Page at the end of you title page.
   (Do not add a Page number.)
   
   
   (Or, go to Insert Menu and choose Section Break next Page.)
   
2. Switch to the second page, then to Header or Footer.
   (Where ever you intend to add Page Number.)
   
3. While in the header or Footer switch to Header-Footer Tab and deactivate "link To Previous".
   
   (Follow directions in the annotation.)
   
4. Or you can go to Format menu → Document
   
   (Then Click on Layout.)
   
   (Now Follow directions in the annotation)
   
5. Then Turn off "Link to Previous."
   
6. Once "Link to previous" is inactive, then add you page numbers.

Bob's method is much easier. But should it not work for you then I submit these alternates to try.

Thanks all - I've managed to fix the problem. Bob's original reply concerning the margins made me look at the tab stops. I simply dragged the tab to the edge of the text and it now works fine.