Stumbling block #1 was needing the -BasedOnReleaseVersion=0.1 flag in the cooker settings for the DLC profile.
Somewhere in proceedings I also setup a profile for cooking my project with a version 0.1 - so I’m not sure if this is needed or not.
Now the cooker appears to do some work, though it ends with an error on launch and doesn’t actually seem to create a .pak file (small issues … )
Stumbling block #2 I’m stuck on right now - trying to work out how to get the DLC builder to package loose content (json files etc) within the DLC structure. For the main game this is done in the package settings. I’m also not sure how to author any content within the DLC directory in my Plugins subfolder.