Thanks Arnoud, for identifying the location of the XML-generation file. I had hunted around for a plain XML file; it was not there, so clearly it was being generated by code.
But this is a really annoying failure by YOAST. Surely the choice of daily or weekly sitemap updates ought to be a user setting ... not something that Yoast chooses on our behalf!
We were making massive changes to our site or an hourly basis for the first few weeks - hundreds of hours of work - but found that Google was simply not indexing the changes for an unacceptably long time (i.e. around a week). At the end, the reason was obvious: this ridiculous decision by Yoast to fix the sitemap change frequency to weekly.
We have implemented the changes as suggested by Arnound, but personally I would prefer to manage my own XML file. Does anyone how to do that correctly, without causing interference or conflicts with the Yoast file? E.g.
* Do we just add our own sitemap.xml at the root level?
* Should we delete the Yoast PHP file that generates the sitemap ( /wp-content/plugins/wordpress-seo/inc/class-sitemaps.php) or can we leave it there?
In the meanwhile, Yoast, please fix this!