Thanks very much for offering. I'd love to, but the problem infuriatingly disappeared when I removed the content to send you a sample.

I have checked the headers on two pages that have the problem, and the header text is the same paragraph style, with the same spacing before and after. In addition, the 'header from top' settings are the same for both headers. Anything else it could be?

Edit: from the position of the horizontal dashed line when I'm in H&F mode, the spacing below the header is uniform - but the text outside the header has inconsistent spacing above it.

Edit edit: I deleted the text immediately following the header, in case there was a problem with that paragraph style, but the text that followed it (a different paragraph style) was also inconsistently spaced below the header, in what looks like the same manner.