Tristan - I did get a reply from DxO Support - who were very helpful and quick by the way. This is the info they gave me:
"...The sidecar files generated by Optics Pro are only readable and usable by Optics Pro. They are used to track changes to original image files that you may make from session-to-session. They are also useful if you want to move your processing to another computer system.
All processing now in Optics Pro is included in the program database. This includes projects. So, yes, when you work on a project, it is included in the database file. When backed up and restored, it will still allow you to create a TIFF, DNG or JPG output file. Optics Pro does not touch the original file, be it RAW or JPG. So, it is not lost. However, if you restore a database with projects, the database remembers the original locations of the files you use in the program. So if you moved the files, then the restored project can have a hard time re-establishing the connection to the original file..".
... Just in case anyone else is interested!
When words become unclear, I shall focus with photographs. When images become inadequate, I shall be content with silence. - Ansel Adams