Well, I'm sure it's not the best way to do this...
...but what I did manage to get to work was to save my Shell .blend file to another 'working' version and set all of the page_num values to 0. When I export this as a complete Age with a spawnpoint everything works fine and I can view my Shell within URU after running it through the normal process in UAM.
Best thing I think then is for me to develop my Shell on a separate layer and then transfer it across to my proper Shell .blend file when I'm ready to release it.
-: D'eux :-