@alan-rutter Makes perfect sense to me, I have a very similar scenario -- building a React SPA with a custom .NET backend. Unfortunately there seems to be very little documentation for building a custom backend.
Have you looked here?
https://github.com/FusionAuth/fusionauth-javascript-sdk-express#server-code-requirements
That's what you and I both need to implement in our .NET backend. Then you should be able to use the Angular SDK to connect to it. It's just a bit irritating that there's not already a reference implementation for this.