@john.adey
I suggest you engage with whomever manages the relationship between your company and Thomson Reuters from Thomson Reuters side. It's practically impossible to help you through this forum, since apparently you're not using anything that's officially released and supported by TR. To help you one will need to know the details of how this project came about, where the SDK came from, some tech details of what you're building, how your app is distributed if you're not part of App Studio program and so on and so forth. All of this calls for direct engagement rather than through a generic Q&A forum.

@john.adey
I must say I'm rather puzzled by your question. I've never heard of IAppNavigation interface and none of the members you mentioned look familiar to me. They're certainly not part of ThomsonReuters.Eikon.Integration namespace, which is what I'd expect you to use. To make your App Studio app developed with Eikon .NET SDK subscribe to context broadcast by linked apps you need to use IAppHost.ContextService.Subscribe() method as described in the following tutorial:
https://developers.thomsonreuters.com/app-studio/app-studio-net-sdk/learning?content=4102&type=learning_material_item

@Alex Putkov.

The interface in question is within the 'ThomsonReuters.Eikon-4.0.Integration.1.2.1\lib\net40\ThomsonReuters.Container.Integration' dll, namespace 'ThomsonReuters.Container.Integration'.

I think some of the confusion may be because my question was moved to the 'App Studio' forum, our application is using the ThomsonReuters.Desktop.SDK.

Sorry for the confusion. So, you're not part of App Studio program then, right? And you have a Windows desktop application that runs outside of Eikon, but needs to share context with Eikon apps? In this case you need to use this API:
https://developers.thomsonreuters.com/eikon-apis/side-side-integration-api
The .NET interface you mention doesn't look to me like anything that's ever been officially released. Did you get it as part of EAP or some such some years ago? None of the Eikon .NET SDK packages available on NuGet include the dll you mentioned.

The packages were supplied directly from TR some while ago to ourselves, these weren't publilically available at the time as far as I'm aware.

Our App is runnning within Eikon, and we are wanting to enable interaction with other apps running in Eikon, so I don't beleive the side-side integraiton api will help us. We have existing apps within Eikon which have the ability to receive context from other Eikon apps already, but following the same approach (implementing IAppNavigation) as them does not work, I am wondering if there is a filter or some such set up on your side which you also need to enable/disable?