Edwin could you confirm if the mastersite folders and directories should use the standard recommended Joomla file/directory permissions, on the server please. for some reason mine currently are not.
Thanks

What do you mean by recommende Joomla File/Directoy permissions.

From my point of view, this is server dependent and not Joomla dependent.
For example cPanel with SUPHP forbid using 777 for the directories.
In general, they use 755 but it is also possible to make it work with 750 when managing correctly the ownership of the files and folders.

For the slave site, what is required is to be able follow the symbolic links.
This is what is deined in the .htaccess provided by Joomla.

Some server may use the open_basedir
In this case, you need to provide the permission to access the master from the slave.