Finally fixed.
Using your own account is just mental as clearly the solution is opaque.
- I created a new PC user, called 'Dadmin', opted to NOT associate it to a microsoft email.
- Created a unique password for it
- Updated 'Dadmin' to be the a windows Admin User (within User account management)
- Went to the shared folder - Properties - Security - and added 'Dadmin' as account to have access, and gave it read/write access.
User: INSERT_HOSTNAME\Dadmin
Password: 'oh, you nearly got me'
Now you have a new user account and known password which works. Promise.
And this is prob safer as you're using a spefic account for network access, not your own, so if it's compromised, you can delete, and YOU are not associated to it
Good luck