Once again, thank you Michael for explaining that too me.

I'm beginning to feel that things like that really need to be made either more clearer or made so they can't happen? Perhaps adding a command which will automatically 'smooth' 2 selected points? I know that Align is available but imo this sort of problem shouldn't even come up. Again, I never thought I'd need to be as precise as this using Nurbs.

Ok so I've got my rail sweeped and the shell ready to be Boolean/Unioned together, which I do. I then select the top inside curve to apply a fillet to and... nothing. I can't get the fillet to work? I get the feeling that something isn't lined up correctly or there's a tiny part of the curve I'm unable to select? If so how am I able to get around this?
Here's what I'd like to fillet and I've included to file too.

Just in case people are losing heart in this thread and maybe feel its not worth replying to anymore.. please don't! I'm really trying to understand this and I promise to write up a explanation when all my problems with this are solved. After this I feel I'll have a lot to say that might make future versions better and also help other people understand Nurbs quicker.