Which begs the question as to why the Royal Mail web site sold me postage, and nobody at the post office spotted there was a problem when I posted it...
later - and there appears to be no way to claim for this online. I shall have to send a written claim, I think.
and later still - after some online research I've established this apparently goes back to mid 2017! It's caused by a backlog of mail that has never been cleared, and applies to all mail to Guatemala.
Update 17th March. Apparently Royal Mail now offer service again, as of a few days ago. Doesn't help me, of course...
Also posted at https://ffutures.dreamwidth.org/2154951.html, where there are