It now works reasonably. I know I could have used level transitions, but wanted to keep the "magical" nature of the building - now you walk to a building the size of one of the elemental shrines, can go through one door and see a room the size of the whole building, and then on the other side there's a room even bigger - The effect wouldn't be there if stairs etc. were present I think. Wanted to give it a kind of "magical" feel.
Now all i have to do is to restrict party movement a bit to prevent them from seeing some strange visual things due to the big inside room - but that should work no problem

Thanks again everyone,
Sharkeem