Thanks Paul. 'Different odd & even' was ticked (because I thought I needed that for the page numbers to mirror in the doc that this is a test file for) and Header is 1.25cm. I unticked 'Different odd & even', deleted the page break, saved the file and inserted a new page break and it's still the same. 'Mirror Margins' is ticked and top/bottom/left/right/gutter are set. Section break is set to 'Continuous' but there aren't any in this file.