Update: Restarting Elements fixed it.
I had a few pages set up, each with a gallery on it. All worked fine. I opened the project this afternoon and a lot of the images have disappeared. The filename still appears in the Resources tab but there is no image.
So my question is, do the images need to stay in the same place they were imported from or are they copied to the project subfolders?
Slightly more worrying, some of the images are still visible
If I copy the Site Resources folder to my home directory I can see some of the images are there with GUID filenames