I've adjusted the post-5B/5C split way to link.  While personally I feel like going straight feels like "staying on, get off at the next exit", I see the point that it is signed 5C to continue straight.  Thus I think the nav program issue is that exit refs are designed to be where the exit splits from the motorway, and that's just not the reality here; those refs should really be relations sort of like turn restrictions. 

I'm calling this good enough as anything further is debatable.