Thought I'd share the answer to this problem in the hopes its helpful to someone else...

The problematic certificate was localhost.crt created under the Trusted Root Certification Authorities node in certmgr. I was deleting this certificate and allowing IIS Express to recreate it within Visual Studio as part of troubleshooting however what I did realize was that the certificate was being created each time with an expiry date that had already passed. I have no idea why. I uninstalled IIS Express 10 and reinstalled it (it was the same build), deleted the localhost.crt file again and reran my project from Visual Studio. This time the certificate was created with an expiry date of 2030 and the Excel add-in runs as expected.