So, we have since resolved the issue and I hope the following helps other developers. The app is developed in Xamarin using Visual Studio for Windows. In an attempt to find an alternative way to get testing, we decided to publish to the App Store and use TestFlight. Rather than use VS to publish, we used "xcrun altool ....." from the Mac Terminal application to perform the upload. This then returned a series of issues with the build relating to build configuration (within VS) and the Identifier for the App on the App Store. After correcting these, creating a new provisioning profile (due to updating the Identifier), we successfully submitted the app to the App Store for review. We then created a new Ad Hoc provisioning profile, rebuilt the app within VS, performed archive and distributed Ad Hoc, and the devices accepted the build!