For anyone else who has any similar issues in future.
We managed to resolve the issue by re-generating visual studio project and performing a re-build of the project via Visual Studio.
This needs to be done every time you pull a new update/branch via git.
Additionally, adding the missing flag to the interface header ensures that SpeckleConverter is available as a type in all blueprint contexts. This change will be included in future releases of the Unreal connector (and available now from main branch on github)