Hello,
Once you create a phone layout in muse redirection is applied on the Domain. In your case redirection is applied on benstancombe.businesscatalyst.com. (With in Muse).
As you already have a redirection applied from www.benstancombe.com to benstancombe.businesscatalyst.com redirection codes created in Muse is not going to work.
Here is the solution for you.
1. If you are going to host your site on a third party host then upload you site using "Üpload to FTP Host" Feature. refer to the link for more details
Adobe Muse Help | Uploading an Adobe Muse Site to a third-party hosting service
2.If you are hosting your site in Business catalyst then instead of placing the redirection from www.benstancombe.com to benstancombe.businesscatalyst.com just add domain to your BC site as mentioned in the link below
Adding a domain name to your site and taking site live
Please let me know in case of any further clarification.
Regards
Vivek