Many many thanks for all the help guys. I feel like a whole new world is opening up for me.

A quick question for anyone else still reading the thread. Can you only Boolean closed objects? I tried to union boolean the outer shell I created with the sweeped circle and all that was left behind was the sweeped circle? Should I have closed the main shell for it to work?

Right hold up I'm experiencing problems again. I've got the shell exactly how I want but how do I close it? I thought I'd simply create another curve cage but make its thickness a little smaller than the original. Ho there's numerous problems with this.
1) Copying and pasting the cage and moving it to the side to alter. How do I then get it back to exactly the same position? Object snape doesn't seem to work?
2)Because I can't get it back to the original position exactly it's out of place. When I do a Join command on the 2 shell pieces nothing happens? I'm still left with 2 pieces?

Here's an illustration of what I mean. How do I get these 2 componant to be snapped together exactly.