[mornington]

I'm an idiot. The previous script was set to always completely rebuild the kindle collections (I'd left a debug setting in by accident).

The attached version works as advertised.

Also, I noticed that if you set calibre to "automatically manage" metadata, as suggested, then you may not be able to see your collections in the "device" window in calibre.

No idea why, but the Kindle can see them, and that's the important thing (and you can set either of the other metadata management options in calibre if you really want to see them there, but if you do then make sure you force calibre to refresh the metadata on your kindle, as described earlier, before running the script).