Hi James,
According to your description, you got an error when you want to change master page.
It seems to be a permission error.
If you are not a site collection administrator, please try to add your account to site settings-> site collection administrator.
If this couldn’t work, please use SharePoint designer to change the master page:
https://www.youtube.com/watch?v=o7GM6Fnz_H4
Here are some tips about the error:
http://www.sharepointdiary.com/2014/01/sorry-this-site-hasnt-been-shared-with-you-sharepoint-2013.html
In addition, you could check ULS logs for more information:
Logs file path: C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\LOGS
Best Regards,
CY
TechNet Community Support
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