I’ve partly found an answer – if I select a photo and then select “revert to metadata on disk” then it loads the updated metadata. The only problem is that you’ve got to know what images have been updated – if I tried selecting all images, or just a large folder, and then selected “revert to metadata on disk” then F Stop crashed every time.

So I’m still looking for a way to rescan (ie re-read all the metadata) for ALL images. Surely there must be an easy way to do this without uninstalling and reinstalling F Stop?