Just an update - I finally seem to have gotten to the bottom of this. It seems there were problems with the files in my /Mods/GenericMod/ that I was using as the basis for some of these files. I deleted all of these files in the root directory of my mod and started again. I made the Level, GameMode BP, GameState BP and PrimalGameData BP from scratch following the instructions at this link:
And hey presto, now it seems to be working for my users (as of v0.32).
Thanks for the replies - it really helped. I also have to say a huge thanks to TyoAtrosa for helping me test my mod properly (and confirm the fix worked) along with jslay who took time to go through all my mod settings with me.