Cannot Upload new World to Realms on Windows 10 Bedrock Codebase

Apologies for the private Jira - but my gamertag is visible. 
I cannot upload any worlds to Realms. I have performed all the steps I would normally take as troubleshooting. 

I've flicked through AppData for Minecraft and found the temp world file for upload but I can't find any settings that could be blocking export - I've included comments in the .7Z'd Windows Step Record attached. (EDIT: I tried to attach - could not, please let me know if you'd like the MHTML file.) EDIT 2: Now uploaded

The world itself is around 6.5MB - so no issue with size, my internet is OK, connection OK, I'm signed in, I can access the realm, I can enter normal gameplay etc. 

Troubleshooting steps include (not limited to):
Rebooting Win10
Deleting Minecraft and reinstalling.
Deleting app data folders for Minecraft. 
Signing out and back in. 
Monitoring network usage via WireShark - doesn't seem to be any when Minecraft says it's uploading!
Resetting the Realm. 
Changing World/Realm names.

Firewall amendments (just in case); Port forwarding, putting PC in DMZ temporarily etc. 

Removed every trace of Minecraft from my system manually, and reinstalled.

Still no dice.

Thanks in advance